This commit exports the VarIntSerializeSize function to provide callers
with an easy method to determine how many bytes it would take to serialize
the passed value as a variable length integer.
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).
