clint: Add rules that forbids spaces after a cast operator

https://neovim.io/develop/style-guide.xml#Horizontal_Whitespace

Note that this errors out for e.g.

    if (has_mbyte) mb_copy_char((const char_u **)(&f), &t); \

(found in macros.h:151) or

    #define ECMD_SET_HELP   0x02    /* set b_help flag of (new) buffer before

(found in ex_cmds.h:11) (note `(new) buffer`). I left this as-is because these 
pieces of code are already caught by another rule (braces near if and `/*`-style 
comments). Other false positives I found were:

1. `FUNC_ATTR_NONNULL_ARG(1) FUNC_ATTR_WARN_UNUSED_RESULT`: rejected by 
   requiring type name to start with `[a-zA-Z_]` (this makes `1` not be 
   incorrectly recognized as a type).
2. `#define FOO(var) (var)`: rejected by creating `cast_line`.
3. `(expr) * (expr)`: rejected by constructing regexp so that unary operators 
   require no spaces after them.
4. `int f(void) FUNC_ATTR_PURE`: rejected by requiring line to start with 
   a space.
5. `"." STR(NVIM_VERSION_PATCH) NVIM_VERSION_PRERELEASE`: not rejected, such 
   thing should be rare and it would be easy to get rid of the false positive by 
   e.g. breaking line.

Struct literals like `(MyStruct) { 4, 2 }` are not checked:

1. I am not sure whether they are under this rule at all (this is not a cast).
2. It would be hard to get rid of false positives (like the example with `if` 
   above, but now for *valid* `if() {}`s).

src/clint.py

@@ -219,6 +219,7 @@ _ERROR_CATEGORIES = [
     'whitespace/tab',
     'whitespace/todo',
     'whitespace/line_continuation',
+    'whitespace/cast',
 ]
 
 # The default state of the category filter. This is overrided by the --filter=
@@ -2513,6 +2514,15 @@ def CheckSpacing(filename, clean_lines, linenum, nesting_state, error):
               'Must not indent preprocessor directives, use 1-space indent '
               'after the hash')
 
+    cast_line = re.sub(r'^# *define +\w+\([^)]*\)', '', line)
+    match = Search(r'\((?:const )?(?:struct )?[a-zA-Z_]\w*(?: *\*(?:const)?)*\)'
+                   r' +'
+                   r'-?(?:\*+|&)?(?:\w+|\+\+|--|\()', cast_line)
+    if match and line[0] == ' ':
+        error(filename, linenum, 'whitespace/cast', 2,
+              'Should leave no spaces after a cast: {!r}'.format(
+                  match.group(0)))
+
 
 def GetPreviousNonBlankLine(clean_lines, linenum):
     """Return the most recent non-blank line and its line number.