Avoid allocations on memory operations

[arkpar]

Fixes #101

[pepyakin]

 name               master-a605175ab.bench ns/iter  ark-allocs-73fbccaa4.bench ns/iter  diff ns/iter   diff %  speedup
 bench_regex_redux  9,525,512                       7,109,347                             -2,416,165  -25.37%   x 1.34
 bench_rev_comp     12,549,005                      8,504,604                             -4,044,401  -32.23%   x 1.48
 bench_tiny_keccak  5,654,512                       4,880,091                               -774,421  -13.70%   x 1.16
 fac_opt            51,511                          50,128                                    -1,383   -2.68%   x 1.03
 fac_recursive      54,433                          53,466                                      -967   -1.78%   x 1.02
 recursive_ok       1,435,432                       1,363,597                                -71,835   -5.00%   x 1.05
 recursive_trap     180,653                         177,061                                   -3,592   -1.99%   x 1.02

[pepyakin]

Cool, thanks!