Implement Enumerable#value_counts in C
Implement Enumerable#value_counts in C

C version achieves 1.4x faster computation then pure-Ruby with MJIT:

```
$ RUBYOPT=--jit be benchmark-driver bench/enum_value_counts.yml
Warming up --------------------------------------
              inject     5.160k i/s -      5.346k times in 1.036123s (193.81μs/i)
       unsort_keepna     3.931k i/s -      4.125k times in 1.049343s (254.39μs/i)
       unsort_dropna     4.018k i/s -      4.290k times in 1.067579s (248.85μs/i)
         sort_keepna     3.641k i/s -      3.750k times in 1.029905s (274.64μs/i)
         sort_dropna     3.966k i/s -      4.312k times in 1.087132s (252.12μs/i)
  norm_unsort_keepna     3.928k i/s -      3.960k times in 1.008232s (254.60μs/i)
  norm_unsort_dropna     3.949k i/s -      3.960k times in 1.002719s (253.21μs/i)
    norm_sort_keepna     3.867k i/s -      4.004k times in 1.035546s (258.63μs/i)
    norm_sort_dropna     3.856k i/s -      4.202k times in 1.089626s (259.31μs/i)
     sort_asc_keepna     3.926k i/s -      3.950k times in 1.005991s (254.68μs/i)
     sort_asc_dropna     3.886k i/s -      4.191k times in 1.078601s (257.36μs/i)
norm_sort_asc_keepna     3.960k i/s -      4.081k times in 1.030615s (252.54μs/i)
norm_sort_asc_dropna     3.820k i/s -      3.874k times in 1.014206s (261.80μs/i)Calculating -------------------------------------
                      1.1.0.dev        HEAD
              inject     5.315k      5.323k i/s -     15.478k times in 2.912167s 2.907726s
       unsort_keepna     4.046k      5.868k i/s -     11.793k times in 2.914914s 2.009669s
       unsort_dropna     4.036k      5.731k i/s -     12.055k times in 2.986689s 2.103623s
         sort_keepna     3.947k      5.420k i/s -     10.923k times in 2.767573s 2.015350s
         sort_dropna     3.964k      5.404k i/s -     11.899k times in 3.001654s 2.201731s
  norm_unsort_keepna     3.956k      5.706k i/s -     11.783k times in 2.978188s 2.064953s
  norm_unsort_dropna     3.934k      5.795k i/s -     11.847k times in 3.011254s 2.044224s
    norm_sort_keepna     3.799k      5.352k i/s -     11.599k times in 3.053044s 2.167209s
    norm_sort_dropna     3.713k      5.474k i/s -     11.569k times in 3.115899s 2.113344s
     sort_asc_keepna     3.887k      5.530k i/s -     11.779k times in 3.030595s 2.129854s
     sort_asc_dropna     4.011k      5.587k i/s -     11.656k times in 2.906199s 2.086428s
norm_sort_asc_keepna     3.795k      5.556k i/s -     11.879k times in 3.130437s 2.138072s
norm_sort_asc_dropna     3.808k      5.579k i/s -     11.459k times in 3.009340s 2.054012s

Comparison:
                           inject
                HEAD:      5323.1 i/s
           1.1.0.dev:      5314.9 i/s - 1.00x  slower

                    unsort_keepna
                HEAD:      5868.1 i/s
           1.1.0.dev:      4045.7 i/s - 1.45x  slower

                    unsort_dropna
                HEAD:      5730.6 i/s
           1.1.0.dev:      4036.2 i/s - 1.42x  slower

                      sort_keepna
                HEAD:      5419.9 i/s
           1.1.0.dev:      3946.8 i/s - 1.37x  slower

                      sort_dropna
                HEAD:      5404.4 i/s
           1.1.0.dev:      3964.1 i/s - 1.36x  slower

               norm_unsort_keepna
                HEAD:      5706.2 i/s
           1.1.0.dev:      3956.4 i/s - 1.44x  slower

               norm_unsort_dropna
                HEAD:      5795.4 i/s
           1.1.0.dev:      3934.2 i/s - 1.47x  slower

                 norm_sort_keepna
                HEAD:      5352.0 i/s
           1.1.0.dev:      3799.2 i/s - 1.41x  slower

                 norm_sort_dropna
                HEAD:      5474.3 i/s
           1.1.0.dev:      3712.9 i/s - 1.47x  slower

                  sort_asc_keepna
                HEAD:      5530.4 i/s
           1.1.0.dev:      3886.7 i/s - 1.42x  slower

                  sort_asc_dropna
                HEAD:      5586.6 i/s
           1.1.0.dev:      4010.7 i/s - 1.39x  slower

             norm_sort_asc_keepna
                HEAD:      5555.9 i/s
           1.1.0.dev:      3794.7 i/s - 1.46x  slower

             norm_sort_asc_dropna
                HEAD:      5578.8 i/s
           1.1.0.dev:      3807.8 i/s - 1.47x  slower

```

mrkn/enumerable-statistics,mrkn/enumerable-statistics,mrkn/enumerable-statistics,mrkn/enumerable-statistics,mrkn/enumerable-statistics