Remove `FromHex` from hash and script types
- Remove the `FromHex` implementation from hash types and `ScriptBuf`
- Remove the `FromStr` implementation from `ScriptBuf` because it does not
roundtrip with `Display`.
- Implement a method `from_hex` on `ScriptBuf`.
- Implement `FromStr` on hash types using a fixed size array.
This leaves `FromHex` implementations only on `Vec` and fixed size arrays.
This renames `Script` to `ScriptBuf` and adds unsized `Script` modeled
after `PathBuf`/`Path`. The change cleans up the API a bit, especially
all functions that previously accepted `&Script` now accept truly
borrowed version. Some functions that perviously accepted `&[u8]` can
now accept `&Script` because constructing it is no loger costly.
This can be replicated by deleting the `type PackedLockTime = LockTime'
line, and then running
find . -type f | xargs sed -i 's/PackedLockTime/LockTime/g
at the root of the repo.