Fix out of date `Scalar` documentation

Scalars can represent integers up to u128, but the docs state otherwise.

compiler/rustc_middle/src/mir/interpret/value.rs

@@ -96,7 +96,7 @@ impl<'tcx> ConstValue<'tcx> {
 }
 
 /// A `Scalar` represents an immediate, primitive value existing outside of a
-/// `memory::Allocation`. It is in many ways like a small chunk of a `Allocation`, up to 8 bytes in
+/// `memory::Allocation`. It is in many ways like a small chunk of a `Allocation`, up to 16 bytes in
 /// size. Like a range of bytes in an `Allocation`, a `Scalar` can either represent the raw bytes
 /// of a simple value or a pointer into another `Allocation`
 #[derive(Clone, Copy, Eq, PartialEq, Ord, PartialOrd, TyEncodable, TyDecodable, Hash)]