reduce number of Unicode private-use characters

This narrows the range of Unicode codepoints fish reserves for its own
use from U+E000 thru U+F8FE (6399 codepoints) to U+F600 thru U+F73F (320
codepoints). This is still not ideal since fish shouldn't be using any
Unicode private-use codepoints but it's a step in the right direction.

This partially addresses issue #2684.

src/reader.cpp

@@ -2964,16 +2964,20 @@ static int can_read(int fd)
     return select(fd + 1, &fds, 0, 0, &can_read_timeout) == 1;
 }
 
-/**
-   Test if the specified character is in the private use area that
-   fish uses to store internal characters
-
-    Note: Allow U+F8FF because that's the Apple symbol, which is in the
-    OS X US keyboard layout.
-*/
+// Test if the specified character is in a range that fish uses interally to
+// store special tokens.
+//
+// NOTE: This is used when tokenizing the input. It is also used when reading
+// input, before tokenization, to replace such chars with REPLACEMENT_WCHAR if
+// they're not part of a quoted string. We don't want external input to be able
+// to feed reserved characters into our lexer/parser or code evaluator.
+//
+// TODO: Actually implement the replacement as documented above.
 static int wchar_private(wchar_t c)
 {
-    return ((c >= 0xe000) && (c < 0xf8ff));
+    return ((c >= RESERVED_CHAR_BASE && c < RESERVED_CHAR_END) ||
+            (c >= ENCODE_DIRECT_BASE && c < ENCODE_DIRECT_END) ||
+            (c >= INPUT_COMMON_BASE && c < INPUT_COMMON_END));
 }
 
 /**