refactor(grid): change schar_T representation to be more compact

Previously, a screen cell would occupy 28+4=32 bytes per cell
as we always made space for up to MAX_MCO+1 codepoints in a cell.

As an example, even a pretty modest 50*80 screen would consume

50*80*2*32 = 256000, i e a quarter megabyte

With the factor of two due to the TUI side buffer, and even more when
using msg_grid and/or ext_multigrid.

This instead stores a 4-byte union of either:
- a valid UTF-8 sequence up to 4 bytes
- an escape char which is invalid UTF-8 (0xFF) plus a 24-bit index to a
  glyph cache

This avoids allocating space for huge composed glyphs _upfront_, while
still keeping rendering such glyphs reasonably fast (1 hash table lookup
+ one plain index lookup). If the same large glyphs are using repeatedly
on the screen, this is still a net reduction of memory/cache
consumption. The only case which really gets worse is if you blast
the screen full with crazy emojis and zalgo text and even this case
only leads to 4 extra bytes per char.

When only <= 4-byte glyphs are used, plus the 4-byte attribute code,
i e 8 bytes in total there is a factor of four reduction of memory use.
Memory which will be quite hot in cache as the screen buffer is scanned
over in win_line() buffer text drawing

A slight complication is that the representation depends on host byte
order. I've tested this manually by compling and running this
in qemu-s390x and it works fine. We might add a qemu based solution
to CI at some point.

src/nvim/map.c

@@ -47,25 +47,6 @@ static inline uint32_t hash_cstr_t(const char *s)
 
 #define equal_cstr_t strequal
 
-// when used as a key, String doesn't need to be NUL terminated,
-// and can also contain embedded NUL:s as part of the data.
-static inline uint32_t hash_String(String s)
-{
-  uint32_t h = 0;
-  for (size_t i = 0; i < s.size; i++) {
-    h = (h << 5) - h + (uint8_t)s.data[i];
-  }
-  return h;
-}
-
-static inline bool equal_String(String a, String b)
-{
-  if (a.size != b.size) {
-    return false;
-  }
-  return memcmp(a.data, b.data, a.size) == 0;
-}
-
 static inline uint32_t hash_HlEntry(HlEntry ae)
 {
   const uint8_t *data = (const uint8_t *)&ae;