implement our own assert() function

I recently upgraded the software on my macOS server and was dismayed to
see that cppcheck reported a huge number of format string errors due to
mismatches between the format string and its arguments from calls to
`assert()`. It turns out they are due to the macOS header using `%lu`
for the line number which is obviously wrong since it is using the C
preprocessor `__LINE__` symbol which evaluates to a signed int.

I also noticed that the macOS implementation writes to stdout, rather
than stderr. It also uses `printf()` which can be a problem on some
platforms if the stream is already in wide mode which is the normal case
for fish.

So implement our own `assert()` implementation. This also eliminates
double-negative warnings that we get from some of our calls to
`assert()` on some platforms by oclint.

Also reimplement the `DIE()` macro in terms of our internal
implementation.

Rewrite `assert(0 && msg)` statements to `DIE(msg)` for clarity and to
eliminate oclint warnings about constant expressions.

Fixes #3276, albeit not in the fashion I originally envisioned.

src/input.h

@@ -4,6 +4,7 @@
 #define FISH_INPUT_H
 
 #include <stddef.h>
+
 #include <vector>
 
 #include "common.h"