Create a new class `ASeq`, inheriting from `AExpr`. This is a new kind of expression, which is a sequence of two `CExpr`s.
This class should implement both the `free_vars` and `interfere` methods, and should be constructed with two `CExpr`s.
The `free_vars` method should return the union of the free variables of the two `CExpr`s.
The `interfere` method should produce the union of the interference graphs produced by the two `CExpr`s.
