request: custom allocator support

[jwdevel]

Internally, kaguya uses a lot of std::string, std::vector, and such, when sending data between C++ and Lua.

I would like to be able to customize the allocator used for that memory.
Currently, I have to hook the system-wide malloc/free functions, which is rather brittle.

Note: this is not the Lua allocator (for inside-the-VM allocations), which is already customizable via the State constructor.