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.

problem: can we have Serde?
solution: we have Serde at home

This by itself is just a change of notation, that could be quickly
merged to avoid messy merge conflicts, but upcoming changes are planned:

- keysets no longer need to be defined in one single file. `keysets.h` is
  just the initial automatic conversion of the previous `keysets.lua`.
  keysets just used in a single api/{scope}.h can be moved to that file, later on.

- Typed dicts will have more specific types than Object. this will
  enable most of the existing manual typechecking boilerplate to be eliminated.
  We will need some annotation for missing value, i e a boolean will
  need to be represented as a TriState (none/false/true) in some cases.

- Eventually: optional parameters in form of a `Dict opts` final
  parameter will get added in some form to metadata. this will require
  a discussion/desicion about type forward compatibility.

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

Done by manual inspection of the output of this script:
grep -r -l -w "bool\|true\|false" * | grep 'c$\|h$' > has_bool
grep -r -l "stdbool.h" * | grep 'c$\|h$' > has_include
grep -F -x -v -f has_bool has_include

- The 'stripdecls.py' script replaces declarations in all headers by includes to
  generated headers.
  `ag '#\s*if(?!ndef NEOVIM_).*((?!#\s*endif).*\n)*#ifdef INCLUDE_GENERATED'`
  was used for this.
- Add and integrate gendeclarations.lua into the build system to generate the
  required includes.
- Add -Wno-unused-function
- Made a bunch of old-style definitions ANSI

This adds a requirement: all type and structure definitions must be present
before INCLUDE_GENERATED_DECLARATIONS-protected include.

Warning: mch_expandpath (path.h.generated.h) was moved manually. So far it is
the only exception.

Uses a perl script to move it (scripts/movedocs.pl)