rewrite abbr function

Rewrite the `abbr` function to store each abbreviation in a separate
variable. This greatly improves the efficiency. For the common case
it is 5x faster. For pathological cases it is upwards of 100x faster.
Most people should be able to unconditionally define abbreviations in
their config.fish without a noticable slow down.

Fixes #4048

doc_src/abbr.txt

@@ -2,54 +2,57 @@
 
 \subsection abbr-synopsis Synopsis
 \fish{synopsis}
-abbr --add word phrase...
-abbr --rename word new_word
+abbr --add [SCOPE] WORD EXPANSION
+abbr --erase word
+abbr --rename [SCOPE] OLD_WORD NEW_WORD
 abbr --show
 abbr --list
-abbr --erase word
 \endfish
 
 \subsection abbr-description Description
 
-`abbr` manipulates the list of abbreviations that fish will expand.
+Abbreviations are user-defined character sequences or words that are replaced with longer phrases after they are entered. For example, a frequently-run command such as `git checkout` can be abbreviated to `gco`. After entering `gco` and pressing @key{Space} or @key{Enter}, the full text `git checkout` will appear in the command line. The `abbr` command manipulates those abbreviations.
 
-Abbreviations are user-defined character sequences or words that are replaced with longer phrases after they are entered. For example, a frequently-run command such as `git checkout` can be abbreviated to `gco`. After entering `gco` and pressing @key{Space} or @key{Enter}, the full text `git checkout` will appear in the command line.
+Each abbreviation is stored in its own global or universal variable. The name consists of the prefix `_fish_abbr_` followed by the WORD after being transformed by `string escape style=var`. The WORD cannot contain a space but all other characters are legal.
 
-Abbreviations are stored in a variable named `fish_user_abbreviations`. This is automatically created as a universal variable the first time an abbreviation is created. If you want your abbreviations to be private to a particular fish session you can put the following in your *~/.config/fish/config.fish* file before you define your first abbrevation:
+Defining an abbreviation with global scope is slightly faster than universal scope (which is the default). But in general you'll only want to use the global scope when defining abbreviations in a startup script like `~/.config/fish/config.fish` like this:
 
 \fish
 if status --is-interactive
-    set -g fish_user_abbreviations
-    abbr --add first 'echo my first abbreviation'
-    abbr --add second 'echo my second abbreviation'
+    abbr --add --global first 'echo my first abbreviation'
+    abbr --add --global second 'echo my second abbreviation'
+    abbr --add --global gco git checkout
     # etcetera
 end
 \endfish
 
-You can create abbreviations directly on the command line and they will be saved automatically and made visible to other fish sessions if `fish_user_abbreviations` is a universal variable. If you keep the variable as universal, `abbr --add` statements in <a href="tutorial.html#tut_startup">config.fish</a> will do nothing but slow down startup slightly.
+You can create abbreviations interactively and they will be visible to other fish sessions if you use the `-U` or `--universal` flag or don't explicitly specify the scope and the abbreviation isn't already defined with global scope. If you want it to be visible only to the current shell use the `-g` or `--global` flag.
 
 \subsection abbr-options Options
 
-The following parameters are available:
+The following options are available:
 
-- `-a WORD PHRASE` or `--add WORD PHRASE` Adds a new abbreviation, causing WORD to be expanded to PHRASE.
+- `-a WORD EXPANSION` or `--add WORD EXPANSION` Adds a new abbreviation, causing WORD to be expanded to PHRASE. You can optionally specify `-g` or `--global` to avoid the overhead of universal variables at the expense of not having the definition being immediately visible to other fish shells that are already running. If you don't specify global scope it default to universal. For clarity you can also specify `-U` or `--universal`.
 
-- `-r WORD NEW_WORD` or `--rename WORD NEW_WORD` Renames an abbreviation, from WORD to NEW_WORD.
+- `-r OLD_WORD NEW_WORD` or `--rename OLD_WORD NEW_WORD` Renames an abbreviation, from OLD_WORD to NEW_WORD.
 
-- `-s` or `--show` Show all abbreviated words and their expanded phrases in a manner suitable for export and import.
+- `-s` or `--show` Show all abbreviations in a manner suitable for export and import.
 
 - `-l` or `--list` Lists all abbreviated words.
 
 - `-e WORD` or `--erase WORD` Erase the abbreviation WORD.
 
-Note: fish version 2.1 supported `-a WORD=PHRASE`. This syntax is now deprecated but will still be converted.
-
 \subsection abbr-example Examples
 
 \fish
-abbr -a gco git checkout
+abbr -a -g gco git checkout
 \endfish
-Add a new abbreviation where `gco` will be replaced with `git checkout`.
+Add a new abbreviation where `gco` will be replaced with `git checkout` global to the current shell. This abbreviation will not be automatically visible to other shells unless the same command is run in those shells (such as when executing the commands in config.fish).
+
+\fish
+abbr -a -U l less
+\endfish
+Add a new abbreviation where `l` will be replaced with `less` univeral so all shells. Note that you omit the `-U` since it is the default.
 
 \fish
 abbr -r gco gch