refactor: replace char_u with char

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

Also add the EXITFREE definition to main_lib rather than the nvim target, as the header generation needs the EXITFREE flag to work properly.

* fix: log and clear error in ui_comp_event

* fix: handling error in each map_foreach_value iteration

* fix: handling error decl in for_each loop

* fix: updating initerr to const, removing initerr free-ing

* fix: using ERROR_SET for error check

* fix: wrapping ERROR_INIT in parens to allow for including inside macro

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

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.

Lua makes (or reuses) an internal copy of strings, so we can safely push
buf pointers onto the stack.

* 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