The import/export operations were available in `integer-gmp-0.5.1`
already, but need to be reimplemented from scratch for the
`integer-gmp-1.0.0` rewrite.

This also adds a few more operations than were previously available for
use w/ the `BigNat` type (which will be useful for implementing
serialisation for the upcoming `Natural` type)

Specifically, the following operations are (re)added (albeit with
slightly different type-signatures):

 - `sizeInBaseBigNat`
 - `sizeInBaseInteger`
 - `sizeInBaseWord#`

 - `exportBigNatToAddr`
 - `exportIntegerToAddr`
 - `exportWordToAddr`
 - `exportBigNatToMutableByteArray`
 - `exportIntegerToMutableByteArray`
 - `exportWordToMutableByteArray`

 - `importBigNatFromAddr`
 - `importIntegerFromAddr`
 - `importBigNatFromByteArray`
 - `importIntegerFromByteArray`

NOTE: The `integerGmpInternals` test-case is updated but not yet
      re-enabled as it contains tests for other primitives which aren't
      yet reimplemented.

This addresses #9281

Reviewed By: austin, duncan

Differential Revision: https://phabricator.haskell.org/D480