Improve readability of lexer debug outputs

[ashmaroli]

Before

start blocks
    pushing :expr_start
lexer: ruby
stack: [:root, :expr_start]
stream: "#######\n# ruby 1.9 e"
  entering mixin inline_whitespace
  trying #<Rule /[ \t\r]+/>
  exiting  mixin inline_whitespace
  trying #<Rule /\//>
  trying #<Rule /
          [?](\\[MC]-)*     # modifiers
          (\\([\\abefnrstv\#"']|x[a-fA-F0-9]{1,2}|[0-7]{1,3})|\S)
          (?!\w)
        /x>
  trying #<Rule /(\s*)(%[rqswQWxiI]? \S* )/>
  entering mixin sigil_strings
  trying #<Rule /%([rqswQWxiI])?([^\w\s])/>
  exiting  mixin sigil_strings
  trying #<Rule //>
    got ""
    popping stack: 1
lexer: ruby
stack: [:root]
stream: "#######\n# ruby 1.9 e"
  entering mixin whitespace
  entering mixin inline_whitespace
  trying #<Rule /[ \t\r]+/>
  exiting  mixin inline_whitespace
  trying #<Rule /\n\s*/m>
  trying #<Rule /#.*$/>
    got "#######"
    yielding Comment.Single, "#######"
lexer: ruby
stack: [:root]
stream: "\n# ruby 1.9 examples"
  entering mixin whitespace
  entering mixin inline_whitespace
  trying #<Rule /[ \t\r]+/>
  exiting  mixin inline_whitespace
  trying #<Rule /\n\s*/m>
    got "\n"
    yielding Text, "\n"
    pushing :expr_start

After

start blocks
    pushing: :expr_start

lexer: ruby
stack: [:root, :expr_start]
stream: "#######\n# ruby 1.9 e"
  entering: mixin :inline_whitespace
  trying: #<Rule /[ \t\r]+/>
  exiting:  mixin :inline_whitespace
  trying: #<Rule /\//>
  trying: #<Rule /
          [?](\\[MC]-)*     # modifiers
          (\\([\\abefnrstv\#"']|x[a-fA-F0-9]{1,2}|[0-7]{1,3})|\S)
          (?!\w)
        /x>
  trying: #<Rule /(\s*)(%[rqswQWxiI]? \S* )/>
  entering: mixin :sigil_strings
  trying: #<Rule /%([rqswQWxiI])?([^\w\s])/>
  exiting:  mixin :sigil_strings
  trying: #<Rule //>
    got: ""
    popping stack: 1

lexer: ruby
stack: [:root]
stream: "#######\n# ruby 1.9 e"
  entering: mixin :whitespace
  entering: mixin :inline_whitespace
  trying: #<Rule /[ \t\r]+/>
  exiting:  mixin :inline_whitespace
  trying: #<Rule /\n\s*/m>
  trying: #<Rule /#.*$/>
    got: "#######"
    yielding: Comment.Single, "#######"

lexer: ruby
stack: [:root]
stream: "\n# ruby 1.9 examples"
  entering: mixin :whitespace
  entering: mixin :inline_whitespace
  trying: #<Rule /[ \t\r]+/>
  exiting:  mixin :inline_whitespace
  trying: #<Rule /\n\s*/m>
    got: "\n"
    yielding: Text, "\n"
    pushing :expr_start

[pyrmont]

@ashmaroli More of a wider question: