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

(The implementation is a bit ugly due to problems I had with Cython templates.  If you can get it to work without crap like WordArrayPtr, please let me know.)

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).