perf: Subtype checking should not perform quadratic traversal

[christoph-dfinity]

The equality check performed at the beginning of subtype checking:

candid/rust/candid/src/types/subtype.rs

Lines 39 to 41 in 30c3886

	if t1 == t2 {
	return Ok(());
	}

should only do a cheap check for "physical" equality (using ptr_eq on anything non-primitive).

Otherwise we do full traversals of complex nested types at every level (quadratic).