[SE-0206] Incorporate acceptance decision notes to proposal text

- Remove Hasher.init(seed:)
- Remove most Hasher.combine overloads
- Change Hasher.finalize to be __consuming
- Clarify that Hasher is order-sensitive
- Clarify hashValue's deprecated status (only custom implementations are deprecated)
- Clarify that SE-0206 is *not* going to be source-breaking; hashValue impls will compile with a warning in 4.2 mode.

Author: lorentey