lang: dispatch operators to the `core::ops` traits

a + b on a user type is a type error. Operators are built in for scalars and tensors, and there is no path from a BinaryOp to a user method.

What needs it. core::ops is a set of definitions that mean nothing until this exists. Wrapping<T>, Saturating<T> and Duration are unusable without it, and == on Option<T> cannot be written at all.

Today. src/hir/check/operators.rs handles scalar and tensor arithmetic directly. Tensors already get operator overloading of a sort -- x @ y and rank-2 * both route to linalg.matmul -- so there is a hook to generalize rather than a mechanism to invent.

Scope. Arithmetic and bitwise operators over Add/Sub/Mul/Div/Rem/BitAnd/BitOr/BitXor/Shl/Shr, comparisons over PartialEq/PartialOrd, and a[i] over Index/IndexMut. Output is an associated type in Rust, so this wants A10 first or a parameter in its place.

Part of the core library plan (#451), item A11. Needed by phase 2.

Start in src/hir/check/operators.rs and inspect how scalar, tensor, and linalg.matmul operators are dispatched. Read the core::ops definitions and the dependency on A10 in issue #451 before choosing how Output is represented. Done means arithmetic, bitwise, comparison, and indexing operators reach the corresponding user traits, including IndexMut.