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.