Round out memory pool allocations in fast-import to pointer sizes.

Round out memory pool allocations in fast-import to pointer sizes.

Some architectures (e.g. SPARC) would require that we access pointers
only on pointer-sized alignments.  So ensure the pool allocator
rounds out non-pointer sized allocations to the next pointer so we
don't generate bad memory addresses.  This could have occurred if
we had previously allocated an atom whose string was not a whole
multiple of the pointer size, for example.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>