Strict mode, serialisation fix, pubkey rare prefixes

Added

• Support for 0x06 and 0x07 pubkey prefixes
• Strict mode

Fixed

• StackData serialisation

Backward compatibility

• many methods that previously expected a check_network flag, now expect a strict flag with the same semantics
• some script_pubkeys that were previously recognised as 'nonstandard' will now be recognised as the appropriate type if strict is set to False