Fix incorrect string length when pushing unicode strings; use an array of chars

Can't use emscripten strings, as you cannot get their length
Don't want to use `char*` because then we have to do an extra unwanted stack save/restore
Fix accidental global
Have `state.load` take an optional chunk name and mode
Have `proxy.invoke` take an array of arguments, so that we can pass extra parameters. Add `n_results`
Be explicit about `state.load` returning a Proxy object
Clean up Makefile more, add custom LUA_API macro to luaconf.h so that we don't need EXPORTED_FUNCTIONS