refactor: replace char_u with char

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

refactor: replace char_u with char

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

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.

Co-authored-by: Marco Lehmann <m99@posteo.de>

Uncrustify struggles to format function-like macros which are defined in
deeply nested areas of the code. Un-nesting them unblocks useful
formatting rules from uncrustify.

* 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.