Previously these RULES were quite fragile as they could compete with the
`unpack` rule defined in `GHC.Base`. This was tickled by 7.10.2, which
happened to rewrite the LHS of rules with other rules (which is itself
a bug in GHC). This caused the literal rules to fail to fire, which then
resulted in long compilation times.

To avoid this sort of fragility, we would like the literal rules to fire
very early in the simplifier. For this reason we set them to rule in
phase 2.

See GHC Trac #10528 for further discussion [1].

[1] https://ghc.haskell.org/trac/ghc/ticket/10528