Export allocate_generic_buffers from main.c

Will be required in tests for proper Neovim initialization.
Add X[MC]ALLOC_NEW convenience macros
Move MENUDEPTH macros to menu.h
Make replace_termcodes and friends accept length and cpo_flags

Reasons:
- One does not have to do `s[len] = NUL` to work with these functions if they do
  not need to replace the whole string: thus `s` may be const.
- One does not have to save/restore p_cpo to work with them.
Add findoption_len function

It is like findoption(), but works with non-NUL-terminated strings.
Add get_option_properties function
Add MAX_CHAR_LEN macros
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.
Do not use lua_*integer functions
Replace all viml moon tests with lua tests

Reason: busted/luajit/whatever enters infinite cycle when using .moon tests.

Reason for entering infinite cycle is not known. This commit fixes tests (at
least, on my machine), but not the bug.

Ref #903
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.
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.
Compile static luajit library with -fPIC

Should fix the following travis error:

    /usr/bin/ld: /opt/neovim-deps/lib/libluajit-5.1.a(lj_err.o): relocation R_X86_64_32S against `.rodata' can not be used when making a shared object; recompile with -fPIC
    /opt/neovim-deps/lib/libluajit-5.1.a: could not read symbols: Bad value
Add code for dumping objects

Untested
Use function that returns top-level state dictionary
Add support for modifying assignments

I.e. let +=, let -=, let .=.
Get rid of indent variables: use lua blocks instead
Add Object dumper for executor tests and test helpers
Start creating executor tests
Add support for v:exception