rules 0.0.2

An implementation of Perl 6 regex rules
Documentation 
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# Differences from Perl 6 by design

Perl uses `#` for comments. Rust uses `//`. Comments in regex
will use `//` instead of `#`. An example:
```rust
r"a // letter `a`
  b // letter `b`
  c // letter `c`
";
```

In perl6, lookaround assertions have `before` and `after` on the
wrong side of the leading to weird things like them both being
backwards when both are used. For example in Perl 6:

```perl
# lookahead
say "foobar" ~~ / foo <?before bar> /; # foo
# lookbehind
say "foobar" ~~ / <?after foo> bar /; # bar
# both
say "foobarbaz" ~~ / <?after foo> bar <?before baz> /; # bar
```

I find this enormously confusing and it throws me every time because
when I see `before`, I *always* interpret it backwards and I have to
manually invert it to comprehend it. I'd invert it like so:

```perl
// lookahead
say "foobar" ~~ / foo <?after bar> /; # foo
// lookbehind
say "foobar" ~~ / <?before foo> bar /; # bar
// both
say "foobarbaz" ~~ / <?before foo> bar <?after baz> /; # bar
```