Rather than using bytes.NewBuffer, which is a read/write entity
(io.ReadWriter), use bytes.NewReader which is only a read entitiy
(io.Reader) in all cases where it is possible. Benchmarking shows it's
slightly faster and it's also technically more accurate since it ensures
the data is read-only.
There are a few cases where bytes.NewBuffer must still be used since a
buffer with a known length is required for those instances.
This commit adds tests for the new SerializeSize functions for variable
length integers and transactions (and indirectly transaction inputs and
outputs).
Both of these depend on the serialized bytes which are dependent on the
version field in the block/transaction. They must be independent of the
protocol version so there is no need to require it.
The go vet command complains about untagged struct initializers when
defining a ShaHash directly. This seems to be a limitation where go vet
does not exclude the warning for types which are a constant size byte array
like it does for normal constant size byte array definition.
This commit simply modifies the tests to use a constant definition cast to a
ShaHash to overcome the limitation of go vet.
