Add a benchmark for symbol-to-proc

Calculating -------------------------------------
               block      5403 i/100ms
              &:to_s      6094 i/100ms
-------------------------------------------------
               block    60023.4 (±5.3%) i/s -     302568 in   5.055537s
              &:to_s    59047.0 (±4.9%) i/s -     298606 in   5.068991s

benchmark/symbol-to-proc

@@ -0,0 +1,6 @@
+require 'benchmark/ips'
+
+Benchmark.ips do |x|
+  x.report('block')  { (1..100).map { |i| i.to_s } }
+  x.report('&:to_s') { (1..100).map(&:to_s) }
+end