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, 1 insertions, 1 deletions

diff --git a/src/nvim/regexp.c b/src/nvim/regexp.c
index 69c2119697..dd7af63ce0 100644
--- a/src/nvim/regexp.c
+++ b/src/nvim/regexp.c
@@ -602,7 +602,7 @@ static int get_char_class(char_u **pp)
   int i;
 
   if ((*pp)[1] == ':') {
-    for (i = 0; i < (int)(sizeof(class_names) / sizeof(*class_names)); ++i)
+    for (i = 0; i < (int)ARRAY_SIZE(class_names); ++i)
       if (STRNCMP(*pp + 2, class_names[i], STRLEN(class_names[i])) == 0) {
         *pp += STRLEN(class_names[i]) + 2;
         return i;