Implement `operator<()`, `std::hash<>`

Bug: b/145351270
Change-Id: I9ebcfccd3a1da36dc9286f6ed6d50c73c8c3bac3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48698
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>