Freeze non-mutated string literals
Freeze non-mutated string literals

Used https://github.com/schneems/let_it_go to find easy places for `String#freeze` optimization.

This will save 885 strings from allocated every request.

Following are from one of my project which uses slim:

  885) /Users/Juan/.gem/ruby/2.2.2/gems/slim-3.0.6/lib/slim/parser.rb
    - 204) String#include? on line 335
    - 204) String#include? on line 334
    - 204) String#include? on line 340
    - 79) String#include? on line 224
    - 79) String#include? on line 230
    - 79) String#include? on line 225
    - 12) String#include? on line 369
    - 12) String#include? on line 370
    - 12) String#include? on line 375

slim-template/slim,ericboehs/slim,JuanitoFatas/slim,raxoft/slim,fly19890211/slim,mojavelinux/slim,cheunghy/slim,ruijsun/slim,JuanitoFatas/slim,cheunghy/slim,slim-template/slim,etdev/slim,fly19890211/slim,raxoft/slim,mojavelinux/slim,slim-template/slim,etdev/slim,ruijsun/slim,ericboehs/slim