Deprecate top-level float

[sampersand]

This PR deprecates the top-level float type, as it's mostly pointless and is not an implicit conversion (like all the other top-level alias types).

Most places you'd expect to use it (eg Math functions) actually require Numeric & _ToF. There's only a few minor edge-cases in the stdlib that use floats directly (eg Regexp.timeout=), but they can instead use _ToF.

[sampersand]

cc @ParadoxV5 , @soutaro , and @ksss . Thoughts?

[ParadoxV5]

Thoughts?

Perhaps we could use a convenience alias for Numeric & _ToF, then.

[sampersand]

It'd only really be used in Math, which already does this with type double = Numeric & _ToF