Make binaryheap available to frontend code.

Make binaryheap available to frontend code.

There are a couple of places in frontend code that could make use
of this simple binary heap implementation.  This commit makes
binaryheap usable in frontend code, much like commit 26aaf97b68 did
for StringInfo.  Like StringInfo, the header file is left in lib/
to reduce the likelihood of unnecessary breakage.

The frontend version of binaryheap exposes a void *-based API since
frontend code does not have access to the Datum definitions.  This
seemed like a better approach than switching all existing uses to
void * or making the Datum definitions available to frontend code.

Reviewed-by: Tom Lane, Alvaro Herrera
Discussion: https://postgr.es/m/3612876.1689443232%40sss.pgh.pa.us