Support string method calls

[csnover]

Technically, to be 100% compatible, this should be calling through a metatable assigned to all strings which has __index pointing to the string global table. This intermediate approach works for all sane use cases without requiring so much work. (It is not possible to set a metatable on a string in PUC-Lua, but it is at least possible to call getmetatable on a string and then mess with it.)