[verifier] Add ID immutability verifier

[lxfind]

Add a verifier to check that we never mut borrow the ID field of a key struct.

[sblackshear]

Couple of other test ideas:

• mutable borrow on id field of struct S { id: ID } is fine
• mutable borrow on other_id field of struct S has key { id: ID, other_id: ID } is fine

[sblackshear]

I think we don't actually need to worry about this case. This bytecode will get generated for (e.g.)

struct S<T> { t : T }

fun foo(s: &mut S): &mut T {
  &mut s.t

Since struct_with_key will already stop you from writing a struct with key whose first field is a generic type, I think we're ok.

I don't mind having the MutFieldGeneric case covered anyway, but wanted to point this out (perhaps worth documenting in a comment or something).

[lxfind]

MutBorrowFieldGeneric bytecode is generated as long as the struct is type templated, not necessarily the field borrowing must be type templated.
For example, if we have

struct S<T> has key {
  id: ID,
  v: T
}

fun foo<T>(s: &mut S<T>) {
  s.id == ...

The above code will also generate MutBorrowFieldGeneric bytecode.

[sblackshear]

Good point! We should probably have a test that exercises an example like this too then.