shared/specifier: use realloc to free some memory after specifier expansion

shared/specifier: use realloc to free some memory after specifier expansion

This is a separate commit only because it actually *increases* memory allocations:
==3256==   total heap usage: 100,120 allocs, 100,120 frees, 13,097,140 bytes allocated
to
==4690==   total heap usage: 100,121 allocs, 100,121 frees, 14,198,329 bytes allocated

Essentially, we do a little more work to reduce the memory footprint a bit. For a
test where we just allocate the memory and drop it soon afterwards, this is not
beneficial, but it should still be useful for a long running program.