Bootstrapping MiniPerl6 in Perl5

- Reduce the amount of nodes required to compile MP6::Grammar::Regex
- Implement the Rul::* nodes using MP6, emit MP6 - that would amount to about 5 or 6 nodes

- Implement the required syntax in MP6::Emitter, such that it can emit Perl 5
- Check that the emitted code can compile MP6::Grammar::Regex

- Check that all MP6 files compile and run properly
- Recompile it several times using itself, in order to check the stability of the Bootstrap

- Freeze the project, because it can be useful to bootstrap other VMs

Limitations of the bootstrapping version:

- hash keys are not autoquoted - always use single quotes.
- there is no precedence of operators - always use parenthesis.
- there is no string interpolation - always use single quotes; "\n" doesn't work.
- constructors are called with ::ClassName( 'arg' => 'val', 'arg' => 'val' )
- 'for' loops need to specify '@{}', as in:  for ( @$a ) {...}
- all statements need a semicolon
- 'use v5' is not supported
- no type annotations
- no 'state', 'constant'

Limitations of the Token compiler bootstrapping version:

- no quantifiers
- no variable interpolation

'Token' routines:

  $grammar.parse( $source, $from_pos ) returns ::Match

Built-in token routines:

- namespace: MiniPerl6::Grammar
- these routines are provided by MP6

  comp_unit() - parses a Class or Grammar
  parse() - parses a whole source file (several Classes)

Runtime token routines:

  space()
  digit()
  word()
  newline()
  not_newline()
  
Runtime subroutines:

  join()
  chars()
  substr()
  say()
  print()

Optional subroutines:

- these are nice to have, but they are optional:

  perl()
  yaml()