Add a basic implementation of load for text chunks

[Aeledfyr]

Implements load (and _G for tests).

load is added as a separate module in the stdlib, with Lua::load_load mirroring Lua::load_io; Lua::core doesn't include load, but Lua::full does.

It matches PRLua's implementation, likely closer than we actually want, since PRLua's load has some quirks:

• load can take a function that returns segments of the text; this must be a function, and cannot be a table with a __call metamethod
• the values returned from the function are concatenated using PRLua's weirdly inconsistent concatenation logic
• load catches errors thrown by the inner iterator function and returns them as parse errors

This implementation currently matches PRLua in all of those semantics. I should probably change this to use table.concat internally for consistency, though it isn't trivial -- the iterator function can return either nil or an empty string to stop iteration, which makes the semantics not map perfectly to a metatable-generic concat system.