Make rationalTo{Float,Double} inline in phase 0

sheafrequested to merge
sheaf/ghc:clc-356 into master

This MR implements CLC proposal #356.

We hold off on inlining rationalToFloat and rationalToDouble until phase 0 to allow constant-folding rules to fire, as described in Note [realToFrac natural-to-float] in GHC.Internal.Float.
However, once we get to phase 0, we should inline them, e.g. to expose unboxing opportunities.

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