Lua should be consistent independent of the way they are produced. pcall in lua
should produce the same sort of error that would be returned by a LuaError
conversion.
The situation is not currently great, because LuaError is not Clone, so passing
a LuaError back into rust will result in the error being "consumed".
There is no single obvious form for a set in lua, and it is not very difficult
to accept a table and convert the sequence values into a set.
Also rename some methods as per discussion.
All lua types should now be at least somewhat usable from rust, if
incompletely so. Any lua value should be readable in Rust as a
LuaValue, pop_value should never panic except in internal logic errors.
kyren