Expand palloc/pg_malloc API

This adds additional variants of palloc, pg_malloc, etc. that
encapsulate common usage patterns and provide more type safety.

Examples:

- result = (IndexBuildResult *) palloc(sizeof(IndexBuildResult));
+ result = palloc_obj(IndexBuildResult);

- collector->tuples = (IndexTuple *) palloc(sizeof(IndexTuple) *
collector->lentuples);
+ collector->tuples = palloc_array(IndexTuple, collector->lentuples);

One common point is that the new interfaces all have a return type that
automatically matches what they are allocating, so you don't need any
casts nor have to manually make sure the size matches the expected
result. Besides the additional safety, the notation is also more
compact, as you can see above.

Inspired by the talloc library.

The interesting changes are in fe_memutils.h and palloc.h. The rest of
the patch is just randomly sprinkled examples to test/validate the new
additions.