Introduce unified Lua ffi.cdef builder

Lua ffi is quite fast and simple mechanism allowing to call C functions directly, without using Lua C API. It is usually faster, sometimes much faster. But ffi requires to define all the C methods again in a Lua file in ffi.cdef function. It is as bad as any other code duplication - when the original API is changed, a one could forget to update Lua files and ffi.cdef arguments. It is especially painful, when ffi C definition is really big.

Interestingly, we have exactly the same issue with module.h file - it is in fact a set of copy-pasted code blocks from some header files. But it is copy-pasted automatically. CMake calls a special routine walking over specified header files and extracting code inside /** \cond public */ and /** \endcond public */ tags.

Goal of this ticket is to introduce a similar mechanism to generate ffi C definitions. API RFC:

Option 1

File box_example.h

/* \cond ffi */

int
box_example1(void);

int
box_example2(void);

int
box_example3(void);

/* \endcond ffi */

File box_example.lua

ffi = require('ffi')
ffi.cdef_file('box_example.h')

Option 2

This option allows to collect C definitions by name, from multiple files.

File box_example.h

/* \cond lua box_example */

int
box_example1(void);

int
box_example2(void);

int
box_example3(void);

/* \endcond lua box_example */

File box_another_example.h

/* \cond lua box_example */

int
box_another_example4(void);

int
box_another_example5(void);

/* \endcond lua box_example */

File box_example.lua

ffi = require('ffi')
ffi.cdef_tag('box_example')

Read module.h and trace the CMake routine that extracts declarations between public tags. Compare that generation path with the two proposed ffi.cdef_file and ffi.cdef_tag APIs, then define how tagged declarations from one or multiple headers should become Lua FFI definitions. Done means the generated definitions stay synchronized with the marked C declarations.