Make ChainWatchInterface::filter_block return only idxes, not refs

Make ChainWatchInterface::filter_block return only idxes, not refs

Instead of making the filter_block fn in the ChainWatchInterface
trait return both a list of indexes of transaction positions within
the block and references to the transactions themselves, return
only the list of indexes and then build the reference list at the
callsite.

While this may be slightly less effecient from a memory locality
perspective, it shouldn't be materially different.

This should make it more practical to generate bindings for
filter_block as it no longer needs to reference Rust Transaction
objects that are contained in a Rust Block object (which we'd
otherwise just pass over the FFI in fully-serialized form).