arbitrary lvalues for named/constraint patterns

[lutzhamel]

Both of the following programs should work,

let q = [1,2,3].
let q@1:%integer = 0. -- constraint applied to list location

assert( q == [1,0,3] ).

and

structure Foo with
  data a.
  function __init__
     with x do
       let this@a:%integer = 1.  -- constraining the modification of the data field
     end.
end

let f = Foo(1).
assert( f is Foo(1) ).

That is we should be able to apply named/constraint patterns to any arbitrary lval structure.

[lutzhamel]

both of the above example programs work now.