Compact serialization for scripts

Special serializers for script which detect common cases and encode
them much more efficiently. 3 special cases are defined:

• Pay to pubkey hash (encoded as 21 bytes)
• Pay to script hash (encoded as 21 bytes)
• Pay to pubkey starting with 0x02, 0x03 or 0x04 (encoded as 33 bytes)

Other scripts up to 121 bytes require 1 byte + script length. Above
that, scripts up to 16505 bytes require 2 bytes + script length.