Add _DynamicStructBuilder.to_bytes() and <struct module>.read_from_memory

to_bytes is not very efficient (it makes one necessary copy and one
unnecessary copy), but it will simplify a lot of use cases.

read_from_memory should be maximally efficient for most input types
(including mmap).
Merge branch 'develop' of git://github.com/amluto/capnpc-python-cpp into amluto-develop

Conflicts:
	capnp/capnp.pyx
Fix problems from merge