acbf23aaa5 Add `is_multisig` helper to Script type (Clark Moody)
Pull request description:
A new `is_multisig` helper method to classify bare multisig output scripts.
The form of a valid multisig script is:
- Pushnum `M`
- <N> pubkeys
- Pushnum `N`
- `OP_CHECKMULTISIG`
`N` must equal the number of pushed pubkeys, and `M` must be less than or equal to `N`.
I've tested this against the RPC output of Core at the block level, checking that the total number of multisig outputs matches.
```
Block 350338, 89 multisig
Block 350340, 29 multisig
Block 350341, 4 multisig
Block 350343, 579 multisig
Block 350344, 48 multisig
Block 350346, 11 multisig
Block 350347, 404 multisig
Block 350350, 127 multisig
Block 350351, 1 multisig
Block 350353, 40 multisig
Block 350356, 13 multisig
Block 350357, 2 multisig
Block 350358, 1 multisig
```
ACKs for top commit:
tcharding:
ACK acbf23aaa5
apoelstra:
ACK acbf23aaa5
Tree-SHA512: b8feeaa8725ac63a658897dac3b303fc8b3d56674d796b14569548124928329993bea45482928d9ce85231f1b5837922af8c0a77b2601a92f88b5e2a9394e97f
f4c83b4d8e