add mrb_strlen_lit which makes _lit macros safer

strlen(3) + string literal is usually optimized but strlen(3) doesn't
check if its argument is really a string literal. This is important for
mruby's _static functions to which some _lit macros are expanded (string
literals have static storage). See comment for some additional info.

remove unnecessary parentheses
Merge pull request #1850 from cremno/safer-lit-macros

add mrb_strlen_lit