Potentially store ScriptExtension as a set of bitfields

[Manishearth]

We can assign each Script value to a bit and store ScriptExtension as a bitfield made of three u64s. Intersection becomes fast, and we can use popcnt to find if something is single script. We can export constants for each script value to make this easy to use.

cc @crlf0710

[Manishearth]

Unknown would be the zero state, Common would be the all-set state, Inherited would be Common with an additional Inherited bit set.

[Manishearth]

To make things even faster we could store the script extension value in the SCRIPTS table as well.

[crlf0710]

Seems a good idea.