A drop-in subset of the hlua 0.4 embedding API, backed by the pure-Rust
lua-rs VM instead of the C library.
The goal is source compatibility: a consumer that does
use lua_rs_hlua_shim as hlua; should compile and run against lua-rs without
touching its own code. The implemented surface is the part exercised by
authoscope — Lua, functionN, AnyLuaValue and friends, LuaFunction,
and StringInLua. It is intentionally not the whole hlua API.
Sandboxing note: lua-rs's runtime opens the full standard library, whereas
hlua consumers typically open only string. This is acceptable for a
compatibility spike but widens the sandbox; restricting the opened libraries
is tracked as future work.