Preprocessor support for stdlib RBSs

Writing RBS files, especially for standard libraries, requires a lot of duplications. (Example: File.open, Pathname#open, Kernel#open, CSV.open, ...) They are sometimes essentially depending types of other methods.

We know the problem but also decided not to extend RBS syntax for this purpose, because most of the cases are standard libraries and we don't want to make the syntax much more complex.

The plan-B is using preprocessors!

Considerations:

• Defining APIs (DSL) for describing types.
  • type_of("File.open").keywords.except(:encoding).merge(chdir: type("String")) ...???
• The compilation to preprocessed files. (We want to ship the preprocessed version for production.)
  • We don't want to do rake precompile every time when developing.
• Can it be public API for end users?

The issue names no files, tests, or entry points. Start by reviewing the standard-library RBS definitions and the examples around File.open, Pathname#open, Kernel#open, and CSV.open. Define the preprocessor DSL, compilation workflow, and public API before implementation; done means duplicated signatures can be generated and preprocessed files can be shipped without requiring a development-time precompile step.