Coconut 2.0 pattern-matching changes

[evhub]

For Coconut 2.0, there will be non-backwards-compatible changes in pattern-matching syntax. Specifically:

• Python-style pattern-matching constructs will become the default (see Integrate Python 3.10 pattern matching #558).
  • Thus, vector(x, y) will need to be written as data vector (x, y) and
  • {"a": a, "b": b} will need to be written as {"a": a, "b": b, **{}}.
• =<expr> will be removed and should instead be written ==<expr>.
• x is int will be removed and should instead be written int() as x or x `isinstance` int.
• With is no longer meaning isinstance, is <expr> will now be allowed in patterns to mean identity comparison.

A --coconut-syntax-version flag should be added to allow the old syntax to help in porting old codebases to 2.0.

[evhub]

This has now been implemented, but it's been done in a way that I actually think will be pretty backwards-compatible, potentially obviating the need for any sort of backwards-compatibility flag. In particular:

• =<expr> and x is int still work, they just print warnings (errors on --strict) and
• vector(x, y) still works, since Coconut will now dynamically check to see whether vector is a data type or not to determine whether to do data matching or class matching by default.

Thus, the only real backwards-compatibility issue here is dict patterns, which will need to have **{} added to them to continue working as they previously did. Seems like it's not worth adding a flag just for that, though.