Implement str.split()

[JeffersGlass]

Adds str.split(sep) to the stdlib. Note that the separator has to be explicitly specified; this does not implement split() which splits on whitespace, which should probably wait until we have default arguments.

Also implements WasmFuncWrapper._to_pylist_str() to get lists back out of wasm tests.

[antocuni]

I haven't reviewed it yet but please note that we DO have support for default args.

However, we don't have str|None yet, so str.split is still not easily typeable (but we could do it with a metafunc)

[JeffersGlass]

we DO have support for default args

Oh! I honestly don't think I knew that... exciting!

Don't sweat renewing this - I'm not sure if I've broken something by changing things in WasmFuncWrapper or if this is exposing a bug in str.__getitem__(slice) that I didn't know about... will do some legwork there