decred: Add input_derive_script.

[JoeGruffins]

Decred shares code with bitcoin and a recent change broke signing stake
based inputs. Add decred specific script construction and fix tests.

This fixes a bug introduced by c9e6ca30c925a213bc92c7b283875d166d3c4f75

This is partly my fault as the test input was flawed and did not have the proper script opcode prefix.

Feel free to take these changes and adjust as you like. I do not need credit. Just hope to get the bug fixed asap.

[JoeGruffins]

@matejcik Hey buddy. Any chance you could have a look at this. Almost a year old. Thanks.

[teknico]

IIUC, this change is needed to stake Decred coins. This is the remaining blocker for me and other Decred holders. We could finally use our Model T’s for Decred, after all these years.

[peterzen]

@matejcik please merge this.

[matthawkins90]

It's wild that this hasn't been reviewed or merged yet.