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

1 files changed, 2 insertions, 2 deletions

diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index aed0484356..a19cb36d12 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -3715,7 +3715,7 @@ ExpandFromContext (
      * right function to do the expansion.
      */
     ret = FAIL;
-    for (i = 0; i < (int)(sizeof(tab) / sizeof(struct expgen)); ++i)
+    for (i = 0; i < (int)ARRAY_SIZE(tab); ++i)
       if (xp->xp_context == tab[i].context) {
         if (tab[i].ic) {
           regmatch.rm_ic = TRUE;
@@ -4155,7 +4155,7 @@ static char_u *get_history_arg(expand_T *xp, int idx)
   static char_u compl[2] = { NUL, NUL };
   char *short_names = ":=@>?/";
   int short_names_count = (int)STRLEN(short_names);
-  int history_name_count = sizeof(history_names) / sizeof(char *) - 1;
+  int history_name_count = ARRAY_SIZE(history_names) - 1;
 
   if (idx < short_names_count) {
     compl[0] = (char_u)short_names[idx];