speedup Script#binary_from_string and Script#to_string

  perftools.rb graphs showed that 48% of the time during parsing/serialization roundtime was spent in Script#binary_from_string

  improving Script#binary_from_string resulted in faster code and only 8.5% time in that method. but showed that now to_string spent 25% in Script#to_string

  improving Script#to_string resulted in faster code and only 8.6% time in that method.

  finishing with this new spent time distribution:

    13.3%  Bitcoin::Script#parse
    13.3%  JSON::Ext::Generator::State#generate
     9.5%  Class#binary_from_string
     8.6%  Bitcoin::Script#to_string
     5.6%  Module#pack_var_int
     5.5%  Digest::Instance#digest
     4.2%  Digest::SHA256.digest
     3.5%  Bitcoin::Util#bitcoin_hash

   http://l.uphnix.de/bitcoin-ruby__parsing_tests__git_tree_fb7ad4368436313292899e0b12f5f006ede7a1ac.svg