Represent mutez using the builtin int63 type
In Coq 8.10, builtin support for int63 has been added. This is exactly the type needed to represent mutez.
This MR:
- changes the representation of mutez
- removes the now unused module "int64bv"
- drops support for Coq < 8.10
- adapts the contract proofs for the new implementation. In particular the proof of vesting_tez has been rewritten.