Disable/warn opt rule for textual parser

[chenyan-dfinity]

when dfx call takes an opt type, the opt subtyping rule essentially turns off the type checking. We need to disable or at least warn about the use of opt subtyping rules in the parser.

[nomeata]

I wonder how useful it is in general to apply subtyping here. Subtyping is meant to be used when A sends a message to B according to an old B.did, so really only should play a role during decoding a message from another party.

When A is a CLI interface that parses a textual argument according to A’s believe of B’s interface, we don’t need to apply full subtyping, we merely need to use B.did to help resolve overloading, and maybe for convenience stuff (e.g. omitting optional arguments).

I don’t see a point in ignoring additional record fields, doing the full opt-field handling etc.