Implement global string functions about brs

[sjbarag]

Same deal as #43: I've marked this as "Good first issue" because we have a pretty good pattern around global functions now. Also note that -- because this is a first issue -- a first-time contributor doesn't have to implement all of these at once. That's a pretty big undertaking. We can always split this into multiple issues to handle families of string functions (e.g. "all the ones that pad strings")