Float / Double matchers should use FloatingPoint protocol

[jeffh]

Since Swift has simplified the crazy protocol change for numerics, Nimble should switch to use the new protocol instead of simplifying it down to Double types.

beCloseTo matcher is the obvious one that needs fixing, but others that explicitly use Double should be updated as needed.

[ikesyo]

beCloseTo

The problem here is that we can't provide DefaultDelta value naturally since that must be a generic value of T: FloatingPoint.