Improve type inference for record projection

[byorgey]

When encountering a record projection r.x, we have to make sure that r has a record type. Previously, if the type of r was not manifestly a record type, we simply emitted an error message like "Can't infer the type of a record projection: r.x". This PR improves the situation in three related ways:

• First, we now expand the type of r, if it turns out to be a type alias or recursive type. This was simply a bug: something like tydef R = [x:Int] end; def f : R -> Int = \r. r.x now works, as it should, but previously it was a type error.
• Second, if the type of r is provably not a record type, we generate an appropriate type mismatch error rather than an unhelpful "Can't infer" error.
• Finally, the "Can't infer" message itself is improved: it now says In the record projection r.x, can't infer whether the LHS has a record type. Try adding a type annotation..

[xsebek]

Thanks for the quick fix! 👍