Infinite loop during typechecking (recursive types)

[yannham]

Describe the bug
Following #270, which has been fixed for usual cases, some terms still make the typechecker loop. The occurs check is not implemented, hence the inference of terms introducing recursive equations crashes.

To Reproduce

nickel typecheck <<< '{g = g 0}.g : Num'

thread 'main' has overflowed its stack
fatal runtime error: stack overflow

Expected behavior
The typechecker should reject such terms without crashing.