PERF: Limit memmove to >= 256 bytes, relax contiguity requirements

As per #3089, for now, I'm putting in a "magic" lower limit for memmove of 256 bytes (32 bytes was the case in the affected tests), which seems to be reasonable from local testing (::dubious::), unless someone on [StackOverflow](http://stackoverflow.com/questions/15554565/forcing-gcc-to-perform-loop-unswitching-of-memcpy-runtime-size-checks) gives me a better idea of what to do.

Also, realized that only the stride in the dimension of the copy matters (i.e. the entire array doesn't have to be contiguous, only the copied subarrays do), so I relaxed that requirement (non-contiguous cases don't seem to be tested in our performance regressions, since they're pretty shallow unfortunately, but they do happen often in practice...this should be addressed by #3114).

Here are the vbench results on the low (improved) end (<90%):

```
series_drop_duplicates_string               0.7474      1.0182     0.7340
frame_multi_and_st                        763.4459   1008.7359     0.7568
reindex_frame_level_align                   1.5111      1.9937     0.7579
reindex_frame_level_reindex                 1.5304      1.9711     0.7764
panel_from_dict_equiv_indexes              34.4702     42.6086     0.8090
groupby_frame_cython_many_columns           6.4044      7.4999     0.8539
read_csv_comment2                          40.2233     45.7705     0.8788
groupby_series_simple_cython                7.5779      8.4962     0.8919
```

and the high (regressed) end (>105%, as there were no cases of >110%):

```
frame_ctor_nested_dict                    108.7902    103.3220     1.0529
stats_rolling_mean                          2.8128      2.6283     1.0702
```

I suspect the last results are just noise.

This is 32-bit Linux GCC 4.6.3, mileage may vary (still haven't set up at 64-bit environment), if anyone else could test this commit too that would be great.

---

**EDIT**

repeat run results:
```
reindex_frame_level_reindex                 1.4861     1.9335     0.7686
reindex_frame_level_align                   1.5578     1.9846     0.7849
panel_from_dict_equiv_indexes              35.0858    42.0521     0.8343
groupby_frame_cython_many_columns           6.5504     7.8101     0.8387
read_csv_comment2                          40.2616    46.1578     0.8723
groupby_last_float32                        7.7265     8.6995     0.8882
```
and
```
frame_reindex_columns                       0.4452     0.4237     1.0507
append_frame_single_homogenous              0.4923     0.4669     1.0543
reshape_pivot_time_series                 292.3670   277.2169     1.0547
join_dataframe_integer_key                  2.8652     2.6741     1.0714
```