Parsing numbers

Again what seems like a minor addition represents months of work and that one long weekend. I've learned that parsing number is a really annoying and complex task.

It became a bit easier when I realized I can use the strength of Parsec to parse the contents of the literals (or just values) passed to tonumber. That didn't really help with the conversion process. This became much more reasonable once I realized that while Lua allows numbers in arbitrary bases, it only does that for integers; floating-point numbers are restricted to base 10.

This means I can delegate the base-10 parsing to existing, good implementations, and just do the base-n integers myself. I still don't particularly like this implementation, but since it lives in its own, isolated module, I can improve it whenever I feel the need for it.

One additional caveat is that I found a bug in the actual parser; it doesn't allow numbers starting or ending with a dot, which got an issue with an apt number #69. This will be solved separately.