c34e3cc7cc Re-write size/weight API (Tobin C. Harding)
73f7fbf520 Add code comments to transaction serialization (Tobin C. Harding)
29f20c1d0b Add segwit serialization constants (Tobin C. Harding)
Pull request description:
Audit and re-write the weight/size API for `Block` and `Transaction`. First two patches are trivial, patch 3 contains justification and explanation for this work, copied here:
```
Recently we introduced a bug in the weight/size code, while
investigating I found that our `Transaction`/`Block` weight/size APIs
were in a total mess because:
- The docs were stale
- The concept of weight (weight units) and size (bytes) were mixed up
I audited all the API functions, read some bips (141, 144) and re-wrote
the API with the following goals:
- Use terminology from the bips
- Use abstractions that mirror the bips where possible
```
Please note, this PR introduces panics if a sciptPubkey overflows the calculation `weight = spk.size() * 4`.
Fix#2049
ACKs for top commit:
apoelstra:
ACK c34e3cc7cc
sanket1729:
ACK c34e3cc7cc.
Tree-SHA512: 4944f652e6e362a282a5731140a9438a82d243a4c646b4627d9046a9f9cf13c476881750d432cfbc6b5fe5de1f0c4c9c44ed4569dac4bc11b55a5db28793803c