Make left hand side of -> parse as a tuple of arguments

[c42f]

This makes argument lists on the left hand side of -> parse as tuples. In particular, this fixes a very inconsistent case where the left hand side previously parsed as a K"block"; we now have:

• (a;x=1)->b ==> (-> (tuple-p a (parameters (= x 1))) b

rather than (block a (= x 1)) on the left hand side.

In addition, the following forms are treated consistently

• a->b ==> (-> (tuple a) b)
• (a)->b ==> (-> (tuple-p a) b)
• (a,)->b ==> (-> (tuple-p-, a) b)

The upside of this is that expression processing of -> syntax should be much easier.

There's one aberrant case involving where which is particularly difficult and still not dealt with:

(x where T) -> b does not parse as (where (tuple x) T) on the left hand side. However, where precedence involving :: and -> is already horribly broken and this syntax will always be awful to write unless we make breaking changes. So I'm too tempted to call this a lost cause for now 😬.

Compatibility shims for converting the SyntaxNode form back to Expr in order to keep Expr stable are included. (At some point we should consider fixing this and deleting these shims because the new form is so much more consistent and would be reflected neatly into Expr form.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.19%. Comparing base (30109d4) to head (9573836).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #522      +/-   ##
==========================================
+ Coverage   96.18%   96.19%   +0.01%     
==========================================
  Files          13       13              
  Lines        4032     4071      +39     
==========================================
+ Hits         3878     3916      +38     
- Misses        154      155       +1     

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