Follow-up on GTF

[adlerjohn]

Should we leave more spacing between script tx fields and create tx fields? I noticed you left plenty of space between the input, output and witness values, but script tx doesn't have much room to grow.

Also is there a way we could assign these imm values to make compatibility checking more efficient using bitmasks?

I.e. if script tx fields fit within a bitmask of 0x00ff and create tx fields fit within a bitmask of 0xff00, then it should be easier to specify which fields are not available.

Right now this seems a little fuzzy by just saying:

Panic If:
The input or output type does not match (OutputChange and OutputVariable count as OutputCoin)

We should be able to specify exactly what is valid or invalid.

Originally posted by @Voxelot in #353 (comment)

[xgreenx]

Done during #514