String[]= benchmark added

also fixed file path in bench_scan.rb
add benchmark for bignum#&  (bench_and.rb)
Merge pull request #2834 from kwleland/bignum_bench_and

add benchmark for bignum#&  (bench_and.rb)
Merge pull request #2831 from kwleland/string_element_assignment

String[]= benchmark added
fix path to file large.txt for benchmark