Rename opcodes::All to Opcode

[tcharding]

I may be missing something about the All type but it seems it can be re-named to Opcode with no loss of clarity.

The first few patches do some other clean ups, the last patch is the meat and potatoes.

[apoelstra]

This all looks good. The All name is a vestige of 2015-era pre-Miniscript code when I would parse script out using classify. Then all the pushes would become enum variants containing &[u8], all the OP_RETURN synonyms would become a single thing, and then all the "real" opcodes would go into the smaller Ordinary enum. And the All type was if I actually wanted all the opcodes.

Today I think classify is still used to separate pushes from the rest of a script (through the Script::instructions iterator), but pretty much all interaction with opcodes happens through the All type.

It might be interesting to look at how Script::instructions is actually used. It may be that we should drop the Class enum and Ordinary enum entirely, and just return a (All, Option<&[u8]>) tuple where the latter is populated for push opcodes.

Anyway concept ACK renaming All to Opcode.

[apoelstra]

ACK ef9f049

[tcharding]

Rebased, no other changes.

[apoelstra]

ACK 63d0fa0

[tcharding]

I believe this can merge, it has one ack two weeks old now, I would argue that it counts as a "trivial API change that no one really cares about" - except @stevenroose who asked me to do this change anyways (out of band).