Implement amap/coerce for Array (re #9796)

Implement an `amap`/`coerce` rule in `GHC.Arr` to match the
`map`/`coerce` rule in GHC.Base.

In order to do so, delay inlining `amap` until phase 1.

To prevent the inlining delay from causing major inefficiencies due to
missed list fusion, rewrite `amap` to avoid relying on list fusion. This
has the extra benefit of reducing the size of the compiled amap code by
skipping the impossible case of an array with a negative size.

Reviewed By: nomeata

Differential Revision: https://phabricator.haskell.org/D471