It is less error-prone than manually defining header guards. Pretty much
all compilers support it even if it's not part of the C standard.

Allow Include What You Use to remove unnecessary includes and only
include what is necessary. This helps with reducing compilation times
and makes it easier to visualise which dependencies are actually
required.

Work on https://github.com/neovim/neovim/issues/549, but doesn't close
it since this only works fully for .c files and not headers.

* refactor: readability-uppercase-literal-suffix

* refactor: readability-named-parameter

* refactor: bugprone-suspicious-string-compare

* refactor: google-readability-casting

* refactor: readability-redundant-control-flow

* refactor: bugprone-too-small-loop-variable

* refactor: readability-non-const-parameter

* refactor: readability-avoid-const-params-in-decls

* refactor: google-readability-todo

* refactor: readability-inconsistent-declaration-parameter-name

* refactor: bugprone-suspicious-missing-comma

* refactor: remove noisy or slow warnings

It's confusing to mix vendored dependencies with neovim source code. A
clean separation is simpler to keep track of and simpler to document.

* refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

* refactor: format header files with uncrustify
* fixup(justin): skip formatting of terminfo_defs.h
* fixup: force winsock2 to be included first
* fixup: simplify disable/enable directive to "uncrustify:off/on"

Retires g:Nvim_color_expr callback.