Use double quotes for word tokens

[ingydotnet]

ie, this:

"foo"

becomes:

/\bfoo\b/

and this:

"foo bar baz"

becomes:

/\b(?:foo|bar|baz)\b/

and maybe, we make double quotes define "tokens" instead of the special
regexes. We still need to think about what tokens are and how they get used in
grammars. Will create different issue for that.

Also we should probably eat leading whitespace on the regexes above. At least
by default. Tokenization semantic variations can be made with new Pegex
directives like: %TokenizationMethod whitespace.