Add support for never type

[heshanpadmasiri]

Currently we don't support never type in the front end (ref). Once we do, we need to

• support functions whose return type is never
• check that expressions of type never only occur in places where the language allows them
• check that types equivalent to never only occur where the language allows them
• update reachability handling to deal with non-returning functions

Function's return type is the return type, if it returns normally. When we say function returns never it means function never returns normally, ie. it always either does not return or panics (example).