core: Add find_key_option_len function
core: Use findoption_len in do_set
core: Split out get_option_properties_idx() function
core: Allow zero-length options

Code that expected NUL-terminated strings allowed them and this behaviour is
actually used.
core: Make get_histtype accept length argument
core: Make get_histtype return HIST_DEFAULT if needed
contrib: Make YCM work with .c.h files
contrib: Fix ycm_extra_conf

fix! Somehow raise ValueError() used for debugging purposes slipped here.
*: Start creating viml-lua translator

Progress so far:
- Slightly more then 50% of commands are covered by parsers.
- Almost no commands are supported by translator (mainly supporting block
  (:while/:if/:for/:try)).
- No commands use Vim API.
- No developer documentation.
- Existing implementation was not replaced.

- Top-level parser was created.
- Top-level translator was created, most of commands not covered by translator
  are expected to work automatically once underlying implementation is added.
- Lua bindings have incomplete types support, and complete operator and
  assignment support.

- Most of used functions that are to be documented are documented.

- Lua bindings do not support integers that do not fit into float: luajit
  appears to have integers, but not visible C function for creating them, lua
  5.3 integers support was not tested.
converter: Do not use lua_*integer functions
scripts: Make it build with GCC and macros-generated declarations

GCC may emit declarations like the following:

    static size_t

    # 42 "this/file.c"
    function(args)

for code like this:

    static FDEF(function)

. Declaration generator script does not expect `# …` inside declarations. This
commit makes it treat such things as comments.
translator/tests: Fix format warning emitted by GCC

`%o` format is only used for unsigned integers. Also add test to make sure that
`-octal` is parsed as `(unary-)((octal))`: otherwise translator will have to add
special handling for leading unary minus.
testhelpers: Add code for dumping objects

Untested
translator/lua: Use function that returns top-level state dictionary
translator: Add support for modifying assignments

I.e. let +=, let -=, let .=.
translator: Get rid of indent variables: use lua blocks instead
testhelpers: Add executor test helpers, including Object dumper
tests: Start creating executor tests
tests: Test for failing list assignments
translator/lua: Add support for v:exception