Allow `parse` to read in non-scalar quantities

The goal of this issue is to get something like the following to work: ```rs let length: Length = parse("1.5 km") ``` This already works for `Scalar`s, and type inference is already powerful enough to pass the `Length` type to the `parse` FFI call, but the parsing-part is challenging. See https://github.com/sharkdp/numbat/pull/796 for details.