Record patterns and exprs should have the same shorthand rules with normal cons

[osa1]

With both positional and named arguments to records we couldn't allow things like (a, b) before, because the type could be (U32, Str) or (a: U32, b: Str), we couldn't tell.

Now that the positional arguments are gone (b7dcf46), we can allow this and infer (a: <type of a>, b: <type of b>).

The idea is that named cons and records should be the same other than the constructor name part.

When I have a Con(a: U32, b: Str), Con(a, b) is allowed in both expr and pat contexts. Similarly the syntax with the constructor part (a, b) should be allowed in both expr and pat contexts, as a record.

(I have some thoughts on potentially removing positional arguments from constructors, but more on this later.)

Changes in the parser should just be revert of the parser parts in b7dcf46. (the commit that removed positional fields in records)