String convert case

This expands the String functionality with `.toLowerCase` and `.toUpperCase` methods. I'm working on the assumption that Espruino's charset support is limited to ASCII, as this only catches A-Z and a-z, respectively.

The tests are simple and don't cover the entire range of characters. If it's deemed necessary I can expand the relevant test suite to cover all bases here.

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I did these because it's something I ran into when experimenting the other day (I found out the hard way that they weren't implemented yet), and I wanted to go for something relatively simply to get my hands dirty. I hope I'll be able to contribute to more interesting / higher priority parts of Espruino soon.