Remove long_u: hashtab: Refactor long_u type.

hashtab.h:
- hash_T: long_u -> size_t.

  In principle, a hash value could thought of as just an unsigned number
  without size semantics (uint32_t or uint64_t).  But it is used as
  index at some places, and so, size_t is also eligible. Therea re some
  places where assignments occur between hash_T and size_t variables, in
  both directions. Therefore, if we define hash_T to be of a type having
  a different width than that of size_t, we will have an incorrect
  assignment somewhere that will require an assert/guard.  So the most
  sensible option here seems to do hast_T to be size_t too.

- hashtab_T.ht_mask: long_u -> hash_T.

  Masks are used to be combined with hash_T values, so they should be of
  the same type.

hashtab.c:
- hash_may_resize(): oldsize: long_u -> size_t.
- hash_may_resize(): newsize: long_u -> size_t.
- hash_may_resize(): newmask: long_u -> hash_T.

src/nvim/hashtab.h

@@ -4,7 +4,7 @@
 #include "nvim/vim.h"
 
 /// Type for hash number (hash calculation result).
-typedef long_u hash_T;          
+typedef size_t hash_T;          
 
 /// The address of "hash_removed" is used as a magic number
 /// for hi_key to indicate a removed item. 
@@ -53,10 +53,10 @@ typedef struct hashitem_S {
 ///
 /// The hashtable grows to accommodate more entries when needed.
 typedef struct hashtable_S {
-  long_u ht_mask;               /// mask used for hash value
+  hash_T ht_mask;               /// mask used for hash value
                                 /// (nr of items in array is "ht_mask" + 1)
-  long_u ht_used;               /// number of items used
-  long_u ht_filled;             /// number of items used or removed
+  size_t ht_used;               /// number of items used
+  size_t ht_filled;             /// number of items used or removed
   int ht_locked;                /// counter for hash_lock()
   int ht_error;                 /// when set growing failed, can't add more
                                 /// items before growing works