Link ANTLR4 Miniscript grammar

[Nuhiat-Arefin]

Adds a README link to the ANTLR4 grammar for Miniscript expressions in the antlr/grammars-v4 repository.

[satsfy]

Concept NACK

From my point of view, this doesn't add value for rust-miniscript users. It reads as promotion for the author's own grammar. The grammar also only covers surface syntax with the type system out of scope, so it's of limited use for real tooling anyway. If anything, it should not be occupying space on the project's main README.md because this has not been a need of rust-miniscript users up until now.

ANTLR also does not support Rust targets. Any use-case I can think of is built on top of rust-miniscript, not within it.

Could you provide some stronger justification for including this antlr grammar?

[trevarj]

Concept NACK from me as well.

We aren't using ANTLR4 for parser generation (or any parser/lexer generator), so I don't really see the correlation.

[apoelstra]

I'm gonna say concept ACK.

It's true that this has nothing to do with rust-miniscript, the Rust library. But this library is also the central hub for Miniscript research in general, since the other main implementation is built into Core and not designed to be so extensible.

[apoelstra]

We may want to move this eventually to an "other resources" section. And I guess we ought to link Dmitry's Alloy implementation too, which actually found some bugs here.

[apoelstra]

@Nuhiat-Arefin can you add an "Other Resources" section, above "Release Notes", and add this there instead of near the top of the README?