Shortcut migrations

[hashedone]

Improvement of #25

It should be always correct to migrate from 1.0 to 1.2 version of the contract by applying migrations one by one: 1.0 -> 1.1 -> 1.2.
However it may be a case, where it is strongly inefficient and we want to support a migration from 1.1, but have a completely separate implementation of migration from 1.0. Example like this:

struct Contract { /* ... */ }

#[contract]
impl Contract {
  #[msg(migrate, from = 1.0, to = 1.2)]
  fn migrate1_0(&self, ctx: MigrationCtx, /* ... */)  -> Result<Respons, ContractError> {
    /* ... */
  }

  #[msg(migrate, from = 1.1, to = 1.2)]
  fn migrate1_1(&self, ctx: MigrationCtx, /* ... */)  -> Result<Respons, ContractError> {
    /* ... */
  }
}

There are two migrations to the same version but from different ones. Sylvia should be able to figure that out and still generate all the migrations for all versions. There always should be only one path to migrate from a specific version, as in this case migration from 1.0 to 1.1 is not needed anymore for our contract. Having obsolete migrations should report a warning as they would never be used.