Hex and string built-ins

[gzanitti]

This PR implements the two built-ins mentioned in issue #106 (%hex(...) & %string(...)). Since it's based on the reimplementation of the backend introduced in PR #133, I'm keeping this PR as a draft until #133 is merged.

The built-in %string(...) puts the full string into the bytecode without regard to length. Please let me know if you want to use a different layout.