Get rid of casting a ptr to a 32 bit value

[dmaclach]

This causes a warning if -Wbad-function-cast is enabled on a build.

Maybe I'm missing something because I can't see why the casts were there in the first place? This passes all the tests.

[nikias]

The casts are there because the original values are uint32_t which are casted like this when added to the array:

ptr_array_add(bplist->used_indexes, (void*)(uintptr_t)node_index);

[nikias]

Merged after rebase with e00615023c2108ba457a9089b900b14bbc9bb73b.