Add an interface for all "add" operations
Use Add interface where appropriate
Add AddConstantToPrimitive template

This covers the six primitive numeric types in a DRY way.
WIP: Remove old AddConstantToPrimitiveType

The code generated version is more complete and covers it now.
OpService: update list of built-in ops
README.md: remove blurb about built-ins

Let's just use the built-ins, because it's shorter and easier to
understand. People who care about extensibility will check the
"create a new op" tutorial and find ij.op().run() anyway.