Add parse<T: Dim>(input: String) -> T

[sharkdp]

For now, this adds the necessary infrastructure to give FFI functions access to the inferred return type (Length) for a given call site:

let length: Length = parse("1.5 km")

It turns out the more challenging problem is to do the actual parsing. Quantities can be arbitrarily complex (-1.234e-5 kg m / s²), so we really want to use the actual parser here. But then we also need to run the type checker and a limited form of the interpreter to get the actual value and unit.

see #773