Implement contracts for records and enums

[yannham]

Describe the bug
Currently, the builtin contracts for row types are not implemented yet. This means that row types are unusable at runtime, since any term which is a Promise or Assume with a row type in the annotation leads to a panic if evaluated.

To Reproduce
Write an Assume or a Promise that is consumed during evaluation:

nickel <<< 'Assume({ {| a: Num |} }, {a = 1})'
thread 'main' panicked at 'TODO implement', src/types.rs:241:41

Expected behavior
We expect contracts to be implemented for enums and records, making the previous code to evaluate without errors.