Make a core version of variable, check them, and introduce dependencies in stmt tycker

[ice1000]

Right now, variable A : B cannot have inter-dependencies.

Would be nice to support

variable A : Type
variable a : A

and using a automatically introduces A.

Note that this needs to be recursive, so

variable A : Type
variable k : K
variable n : G k
variable xs : F A n

and using xs will cause everything else to be introduced.