Fix build-failure regression for base < 4.7

The `$>` operator definition is available only since base-4.7 which
unfortunately wasn't caught before release to Hackage (but has been
fixed up by a metadata-revision)

This commit introduces a `CompatPrelude` module which allows to reduce
the amount of CPP by ousting it to a central location, i.e. the new
`CompatPrelude` module. This pattern also tends to reduce the tricks
needed to silence unused import warnings.

Addresses #1119

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