Better types for opcodes

[vostrnad]

ops.ts exports the list of opcodes as:

const OPS: { [key: string]: number }

That's not a very safe type, as it permits access with any string key. It also doesn't work with code completion.

Can this type be made more specific? It would technically be a breaking change for anyone using this in an unsafe manner, but I believe it's worth it. I'm happy to work on this.

[junderw]

OPS is pretty straightforward, (just as as const at the end) but REVERSE_OPS might be tricky to get to only respond to specific number keys...

Admittedly, when I migrated this library to TypeScript I was a TS beginner, and only feel like I now have a semi-intermediate grasp on it...

PRs are welcome, but I think this won't land until the next major version whenever that is.

[ChrisCho-H]

How about using enum as bitcoin core does?
https://github.com/bitcoin/bitcoin/blob/master/src/script/script.h#L72

[ChrisCho-H]

Made PR for this #2028.
Any review or commit would be grateful!