String.f32 and f64 should be implemented in Pony.

[jemc]

Currently String.f32 and f64 use the strtof FFI call internally.

As discussed on the sync call, it would be desirable for us to implement this in pure Pony instead of relying on the C call for something we can do ourselves in a more transparent, debuggable way.

This change should have no impact on the user, so it doesn't need an RFC - it just needs someone to step up and do the implementation.