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/eval.c b/src/nvim/eval.c
index 71cb83e566..caba4464df 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -6652,7 +6652,7 @@ char_u *get_function_name(expand_T *xp, int idx)
     if (name != NULL)
       return name;
   }
-  if (++intidx < (int)(sizeof(functions) / sizeof(struct fst))) {
+  if (++intidx < (int)ARRAY_SIZE(functions)) {
     STRCPY(IObuff, functions[intidx].f_name);
     STRCAT(IObuff, "(");
     if (functions[intidx].f_max_argc == 0)
@@ -6695,7 +6695,7 @@ find_internal_func (
 )
 {
   int first = 0;
-  int last = (int)(sizeof(functions) / sizeof(struct fst)) - 1;
+  int last = (int)ARRAY_SIZE(functions) - 1;
 
   /*
    * Find the function name in the table. Binary search.