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/macros.h

@@ -152,4 +152,12 @@
 
 # define RESET_BINDING(wp)  (wp)->w_p_scb = FALSE; (wp)->w_p_crb = FALSE
 
+/// Calculate the length of a C array.
+///
+/// This should be called with a real array. Calling this with a pointer is an
+/// error. A mechanism to detect many (though not all) of those errors at compile
+/// time is implemented. It works by the second division producing a division by
+/// zero in those cases (-Wdiv-by-zero in GCC).
+#define ARRAY_SIZE(arr) ((sizeof(arr)/sizeof((arr)[0])) / ((size_t)(!(sizeof(arr) % sizeof((arr)[0])))))
+
 #endif  // NVIM_MACROS_H