Update our built-ins.

[skx]

Yet again our builtins are having an overhaul. Specifically I want to make a bunch of changes:

• Move all the builtins to their own package.
• Split the implementation into a couple of files
  • string.go
  • math.go
• 100% test-coverage for all the builtins
  • This will use the object.Object .. object .. directly, rather than using the interpreter.
  • Which should make testing faster, quicker, and easier.

I'm going to have to change the signature of the builtin-function, sadly, which is a breaking change. But only minorly.

Once this pull-request is complete, and merged it will close #56.

[skx]

• Everything updated to use the new API.
• No new tests added.
  • Could add those separately I guess.

[skx]

50ish% coverage of the string tests - found some issues relating to unicode. Will merge once travis approves and attack them in the morning.