Benchmark additional approach to hash merging

This is rather faster than the next best approach.

                    Rate using_var_merge using_keys using_iter using_splice using_hslice
using_var_merge 123589/s              --       -26%       -29%         -29%         -37%
using_keys      167790/s             36%         --        -3%          -4%         -15%
using_iter      173684/s             41%         4%         --          -0%         -12%
using_splice    174450/s             41%         4%         0%           --         -12%
using_hslice    197481/s             60%        18%        14%          13%           --
ok 1 - var_merge, same result as splice_output
ok 2 - var_merge, same result as keys
ok 3 - var_merge, same result as iter
ok 4 - var_merge, same result as hslice
1..4
Merge pull request #1 from arc/master

Faster hash merge
Merge pull request #2 from doherty/master

Add a benchmark for testing a hash for emptiness
string_trimming
trimming benchmark
Merge pull request #3 from yanick/trim

Common string trimming methods
Add Sereal to serialization benchmark
Merge pull request #4 from tsee/master

Adds Sereal to the serialization benchmark
Compare map-EXPR vs map-BLOCK (no difference)