This commit fixes a panic when deserializing PSBTs in raw binary.
If the key type was SighashType and the value was not 4 bytes long,
the call to binary.LittleEndian.Uint32(value) would panic as the
function expects 4 bytes to parse into a uint32. We now perform a
sanity check that asserts that the value is 4 bytes long.
This is a fix/workaround for a special case that's caused by
https://github.com/btcsuite/btcd/blob/master/wire/msgtx.go#L426.
When a wire format transaction with no inputs is serialized, the wire
package assumes it's a non-witness transaction (as there is indeed no
witness data present).
But when de-serializing the same transaction, the line mentioned above
assumes that for the special case of a zero input length, the
transaction must be in the witness format, which causes the
de-serialization to fail.
The workaround in this commit fixes this special case by just trying
to deserialize the transaction in the non-witness format too.
In this commit, we modify the Extract method to return the transaction
directly as in many cases a user will likely want to write the
transaction to disk, or perform additional validation rather than obtain
the raw bytes directly.
The creator struct really didn't do anything before, as a result in this
commit we move to get rid of it, and create a `New` method as
customarily used in go packages.
