This commit adds two new funtions named ReadMessageN and WriteMessageN
which return an additional paramter for the number of bytes read or written,
respectively.
It also adds tests to ensure the number of bytes read and written are the
expected values both for successful reads/writes and unsuccessful ones.
Closes#6.
This commit adds tests to ensure the new "fast" paths in readElement and
writeElement work properly including proper fallback to the slower
reflection based read/write of the binary package.
This commit adds tests for the new SerializeSize functions for variable
length integers and transactions (and indirectly transaction inputs and
outputs).
