Avoid -Wconversion warnings from direct use of GET_n_BYTES macros.

Avoid -Wconversion warnings from direct use of GET_n_BYTES macros.

The GET/SET_n_BYTES macros are meant to be infrastructure for the
DatumGetFoo/FooGetDatum macros, which include a cast to the intended
target type.  Using them directly without a cast, as DatumGetFloat4
and friends previously did, can yield warnings when -Wconversion is on.
This is of little significance when building Postgres proper, because
there are such a huge number of such warnings in the server that nobody
would think -Wconversion is of any use.  But some extensions build with
-Wconversion due to outside constraints.  Commit 14cca1bf8 did a disservice
to those extensions by moving DatumGetFloat4 et al into postgres.h,
where they can now cause warnings in extension builds.

To fix, use DatumGetInt32 and friends in place of the low-level macros.
This is arguably a bit cleaner anyway.

Chapman Flack

Discussion: https://postgr.es/m/592E4D04.1070609@anastigmatix.net