Auto-recover constructor expressions when assigning to an explicit type.

[jemc]

In cases where we are constructing a new object and assigning to that to an explicit type (either as an actual assignment with an explicit type or as an argument passed to a method), and there is a subtyping issue in that assignment which would be solved by surrounding the constructor in a recover ... end, we want to auto-recover the expression and allow that recover ... end to be implicit rather than explicit.

Some examples:

let a: Array[String val] val = ["foo", "bar"]

class Foo
  new create(v: U8) =>
    // ...

primitive Bar
  fun take_foo(foo: Foo iso) =>
    // ...

// ...
Bar.take_foo(Foo(88))

This would also solve #415.