Migrate Operators to Source Generators

[viceroypenguin]

The generated code that we are using would be integrated to the build process better if it were done via SG rather than T4 templates. This is the holding issue for migrating various operators to SG. See https://github.com/viceroypenguin/SuperLinq/tree/master/Generators/SuperLinq.Generator for example of implementation.

• Aggregate
• Cartesian
• Fold
• ToDelimitedString

@atifaziz If you're good with this, feel free to assign this to me, and create subissues and assign those to me as well.