#2099: Fix erroneous caching by providing each function instance with it own interop variant

[warappa]

• Effectively revert delegate cache optimization
  • Each function instance requires its own .NET representation to handle parameters correctly (-> function environment)
• Use a factory method to reduce System.Linq.Expression tree usage and therefore also the need for compiling them.
  • Compile an expression tree that allows to retarget a delegate to the actual Function instance (1 per AST, not per function instance).
  • Ensure returning the same delegate instance to allow for example unregistering handlers
• Add 2 tests
  • Ensure multiple invokations using .NET interop don't cache function environment
  • Ensure multiple invokations using JavaScript-only functions don't cache function environment

[lahma]

Awesome work, great job with test coverage!