Rename `{import,export}Integer`

This renames to more verbose names which include the type these
operations import/export from/to:

 - `importIntegerFromByteArray`, and
 - `exportIntegerToMutableByteArray`.

This follows the naming convention used for other primitive operations,
such as the recently added `copyMutableByteArrayToAddr` operation.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>