One of our stated aims is to make it possible to learn bitcoin by using
our library. To help with this aim add to private consts for the segwit
transaction marker and flag serialization fields.

In an attempt to help super new devs add code comments about transaction
serialization formats pre and post segwit.

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