Replace asserts/fails with lua_errors
Add two comments for Buffer to squelch missing doc warning
Implement argcheck/argerror
Implement pcall()

Tweak the load* methods to return a Result<(),err> instead of
Option<err>.
Implement ToStr for error codes
Add new example sine.rs

Implement a basic REPL loop for examples, that tries to mirror the
built-in Lua one (except without handling incomplete lines).

examples/sine.rs demonstrates exposing a Rust function to Lua.