Breakage due to changes in `String` slicing in v0.7

[stevengj]

@StefanKarpinski

One way to spell the operation in question is this: String(codeunits("∀ x ∃ y")[1:9])

This currently makes an extra copy of the data, but that would be fixed by adding a CodeUnits slicing method that allocates the copy using StringVector (so that String(...) will take ownership of the new array).

This doesn’t help with the SubString case, though.