Need new token for accessing the return value for an interface call

[frame-lang]

Currently returns values are set using ^(ret_value) syntax. However @^ no longer is the universal way to access the return value as the architecture has changed to have self.return_stack. Proposed tokens to access the self.return_stack are:

1. ^?
2. ?^
3. ^()

We also need a way to set the return value w/o executing a return.

1. ^= "foo"
2. ^=("foo")