add missing params to type_check_term_inst

[HalflingHelper]

env, recfun, and subterms were undefined in the method type_check_term_inst.

Here is the file that triggered the error:

import List
import Nat
import Option

theorem rev_correct : all T : type, i : Nat, xs : List<T>.
  get(reverse(xs), i) = get(xs, length(xs) - 1 - i)
proof
  arbitrary T : type, i : Nat
  induction List<T>
  case [] {
    suffices ? by definition { length, get }
    ?
  }
  case node(x, xs') suppose IH {
    ?
  }
end

Gotta love that python lets you have undefined variables floating around.

[jsiek]

Thanks!