Use codepoints instead of chars in the lexer.

Grand wizard overlord @whitequark recommended this as it will bypass the need
for creating individual String instance for every character (at least not until
needed). This becomes noticable on large inputs (e.g. 100 MB of XML).
Previously these would result in the kernel OOM killing the process. Using
codepoints memory increase by a "mere" 1-1,5 GB.

lib/oga/lexer.rl

@@ -95,7 +95,7 @@ module Oga
     # @return [Array]
     #
     def lex(data)
-      @data       = data.chars.to_a
+      @data       = data.codepoints
       lexer_start = self.class.lexer_start
       eof         = data.length
 
@@ -152,7 +152,7 @@ module Oga
     # @return [String]
     #
     def text(start = @ts, stop = @te)
-      return @data[start...stop].join('')
+      return @data[start...stop].pack('U*')
     end
 
     ##
@@ -223,6 +223,7 @@ module Oga
     %%{
       # Use instance variables for `ts` and friends.
       access @;
+      getkey (@data[p] || 0);
 
       newline    = '\n' | '\r\n';
       whitespace = [ \t];