It's modestly faster, significantly more type-safe, and doesn't assert
when there's too few arguments, which causes problems for plural forms.
Closes#1733.
In preparation for switching to LuaJIT, which doesn't support PUC Lua's
thing of using C++ exceptions for lua_error.
Requires replacing all uses of lua_error (and things calling lua_error)
with custom versions that throw an exception instead and adding an
exception -> lua error wrapper at all C++ -> Lua boundaries.
This requires shuffling a bunch of stuff around to get a CLI lua
executable that uses an automation-like environment, but that's
something that'll be nice to have in the future anyway.
Busted indirectly depends on lfs and we currently can't use external
binary modules, so add a copy of lfs to the repo and build it with
Aegisub.
