url parsing and formatting utilities

[odino]

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from @omissis

sounds like we need require('@url') :slightly_smiling_face:

[odino]

I want to take this issue as a way to introduce building native modules in Go -- eg. currently the entire stdlib modules are written in ABS but eventually to expose a bunch of functionalities (like crypto) we should allow doing so in Go. I don't know how exactly we should do this, but the URL example seems an "easier" one.