checker: detect redundant ref when assigning call expr with ref return

[ttytm]

Fixes:

@[heap]
struct Foo {}

fn foo() &Foo {
	return &Foo{}
}

_ := &foo()

throwing:

error: lvalue expected
...
==================
(Use `v -cg` to print the entire error message)

builder error: 
==================
C error. This should never happen.

[spytheman]

Excellent work.