Add primitive for re-generating files from mutations

This MR adds a new primitive for generating a mutant file out of a mutation: Test.save_mutation : mutation -> unit.

Given a mutation m modifying an expression in a file test.mligo, Test.save_mutation m will create a file test.ID.mligo with the mutation applied. The ID is an identifier generated from the mutation (hash of loc + replacing expression => base58 => first 8 chars).

This MR also adds names for constants such as Operator.times for *, Operator.add for +, etc.

• has a changelog entry