Establishing generic parselets and functions

[phorward]

About

This PR is intended to coordinate generic parselets & functions in Tokay as established in #10.

To- do

• [x] Syntax changes
• [x] Handle parselet instances
• [x] Handle generic values in intermediate structures
  • [x] Repeat<P> min=1, max=void
    • [x] ~Think about a replacement of Capture by RefValue with a new value type Capture, so that Repeat<Char> returns a capture range and not a list of values~ turned into #116
    • [x] Implement Pos<P>
    • [x] Implement Opt<P>
    • [x] Implement Kle<P>
    • [x] Compile P+ into Pos<P>
    • [x] Compile P? into Opt<P>
    • [x] Compile P* into Kle<P>
  • [x] List<P, Sep: (',' _), empty: true>
  • [x] Peek<P>
    • [x] Replace peek P by Peek<P>
  • [x] Not<P>
    • [x] Replace not P by Not<P>
  • [x] Expect<P> msg=void
    • [x] Replace expect P by Expect<P>
  • [x] ~Keyword<P>~ turned into #121
    • [x] ~Replace tokay.toks 'kw' _standalone_ by Keyword<'kw'>~
• [x] Tests

[phorward]

This pull request is now ready to be merged.

I'm a little unhappy with the current ImlValue implementation, but this is the only way to adequately solve the problem at the moment. Anyway, this should be done better or differently, and I'm sure it will.

All that's missing now is some documentation to describe the new features.

By implementing build-ins with the help of Tokay itself, #120 will be another big milestone to significantly reduce the size of the generated code and thus make Tokay much more performant again like the previous solution. However, this step here has to be to raise Tokay to another level.