Define and use the ARRAY_SIZE macro

A similar macro is defined in the Linux kernel [1].

To refactor the code I used a slightly modified Coccinelle script I found in
[2].

```diff
// Use the macro ARRAY_SIZE when possible
//
// Confidence: High
// Copyright: (C) Gilles Muller, Julia Lawall, EMN, DIKU.  GPLv2.
// URL: http://www.emn.fr/x-info/coccinelle/rules/array.html
// Options: -I ... -all_includes can give more complete results

@@
type T;
T[] E;
@@

- (sizeof(E)/sizeof(*E))
+ ARRAY_SIZE(E)

@@
type T;
T[] E;
@@

- (sizeof(E)/sizeof(E[...]))
+ ARRAY_SIZE(E)

@@
type T;
T[] E;
@@

- (sizeof(E)/sizeof(T))
+ ARRAY_SIZE(E)

@n@
identifier AS,E;
@@

- #define AS(E) ARRAY_SIZE(E)

@@
expression E;
identifier n.AS;
@@

- AS(E)
+ ARRAY_SIZE(E)
```

`spatch --in-place --sp-file array_size.cocci -I src/ -I build/include/ -I build/src/nvim/auto/ src/nvim/*.c`

[1] http://lxr.free-electrons.com/source/include/linux/kernel.h#L54
[2] http://www.emn.fr/z-info/coccinelle/rules/#macros

src/nvim/syntax.c

@@ -3943,7 +3943,7 @@ get_syn_options (
     if (strchr(first_letters, *arg) == NULL)
       break;
 
-    for (fidx = sizeof(flagtab) / sizeof(struct flag); --fidx >= 0; ) {
+    for (fidx = ARRAY_SIZE(flagtab); --fidx >= 0; ) {
       p = flagtab[fidx].name;
       int i;
       for (i = 0, len = 0; p[i] != NUL; i += 2, ++len)
@@ -6295,7 +6295,7 @@ do_highlight (
         attr = 0;
         off = 0;
         while (arg[off] != NUL) {
-          for (i = sizeof(hl_attr_table) / sizeof(int); --i >= 0; ) {
+          for (i = ARRAY_SIZE(hl_attr_table); --i >= 0; ) {
             len = (int)STRLEN(hl_name_table[i]);
             if (STRNICMP(arg + off, hl_name_table[i], len) == 0) {
               attr |= hl_attr_table[i];
@@ -6416,7 +6416,7 @@ do_highlight (
 
             /* reduce calls to STRICMP a bit, it can be slow */
             off = TOUPPER_ASC(*arg);
-            for (i = (sizeof(color_names) / sizeof(char *)); --i >= 0; )
+            for (i = ARRAY_SIZE(color_names); --i >= 0; )
               if (off == color_names[i][0]
                   && STRICMP(arg + 1, color_names[i] + 1) == 0)
                 break;