Eliminate exceptions from vsubst

Process:
1. Find a function throwing exceptions
2. Make it safe by using Choice.failwith (failing branches),
Choice.bind, etc.
3. Replace "func" by "Choice.get <| func" in NHol project
4. Fix errors which mostly are changing <| to << and adding parentheses
5. Create a commit with all changed files
Process inst function
Eliminate exceptions from rand and rator
Lift safe_mk_eq and dest_eq functions
Review Uppercase functions in fusion module
Cleanup new_axiom and co.
Process new_type and new_constant