refactor: replace char_u with char 16 - remove STRNCMP (#21208)

refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

src/nvim/buffer_defs.h

@@ -318,7 +318,7 @@ typedef struct {
 typedef struct mapblock mapblock_T;
 struct mapblock {
   mapblock_T *m_next;           // next mapblock in list
-  uint8_t *m_keys;              // mapped from, lhs
+  char *m_keys;                 // mapped from, lhs
   char *m_str;                  // mapped to, rhs
   char *m_orig_str;             // rhs as entered by the user
   LuaRef m_luaref;              // lua function reference as rhs