Mutation testing: reduplication for mutation saving

type:fixed

For LIGO developers

Since we are now deduplicating variables in ast-aggregated, this affects the procedure for mutation saving. For example, original code:

let sub (store, delta : storage * int) : storage = store - delta

gets printed after mutation:

let sub (store, delta : storage * int) : storage = (gen__store1330 + gen__delta1331)

This MR fixes the problem by forgetting count in variables when saving a mutation. It also improves printing of mutations by decompiling the expression.

Changelog details: