Improve handling of consume in class constructors

[odersky]

Two changes:

• We now subsume consume parameters in the fresh of a constructor, instead of keeping track of them besides.
• We now treat val _ = e like just the expression e for separation checking.

[bracevac]

Suggested change

	val _: A2^{cap, b} = a2
	val _: A2^ = a2

[bracevac]

Is this intentional? It doesn't seem to matter if it's {cap, b} or {cap}. Or is the point to show that mentioning b explicitly as in A1 above doesn't influence the outcome?

[bracevac]

Trying println(a2.b) at this point yields a separation failure. But why? The b should be owned by a2 now. If this is as intended, it means that consume on a val constructor parameter makes the field inaccessible.

[bracevac]

I'm trying to understand what should happen when the if there is one part is not true.
Say, in the below test file, if we let A1 and A2 have no implied fresh (IIUC):

class A1(val b: B^)
class A2(consume val b: B^)

Then it exhibits the expected errors, plus it complains about
val _: A2^{cap, b} = a2 and println(a2) with the same error message:

Separation failure: Illegal access to (a2 : A2{val b: B^{b²}}^{b²}), which was passed to a
   |consume parameter or was used as a prefix to a consume method on line 18
   |and therefore is no longer available.
   |
   |where:    b  is a value in class A2
   |          b² is a value in method Test

Why is accessing a2 forbidden now?

Independently of whether an error is expected or not: The error message itself is confusing, because a2 neither was "passed to a consume parameter" nor "was used as a prefix to a consume method".