Add benchmark around string replacement.

Calculating -------------------------------------
                 #tr     62416 i/100ms
               #gsub     33750 i/100ms
              #gsub!     29695 i/100ms
                #sub     60774 i/100ms
               #sub!     64955 i/100ms
-------------------------------------------------
                 #tr   989348.8 (±5.0%) i/s -    4993280 in   5.060836s
               #gsub   422892.9 (±4.3%) i/s -    2126250 in   5.037741s
              #gsub!   364115.6 (±4.0%) i/s -    1841090 in   5.064496s
                #sub   964336.6 (±4.4%) i/s -    4861920 in   5.051775s
               #sub!  1016598.5 (±4.7%) i/s -    5131445 in   5.058987s