Coma standard library

The standard library of Coma must be explicitly loaded with use coma.Std.

It includes basic handlers such as:

• halt: no parameters and no VC, can be used to end the program as the “final continuation”
• fail: no parameters but false as precondition, can be used as an assert false
• if: has the prototype (b: bool) (then {b}) (else {not b})