AVX2 transcendentals intrinsics (#329)

Provides the following transcendentals intrinsics:

* `nmc_mm256_exp_pd`
* `nmc_mm256_log_pd`
* `nmc_mm256_pow_pd`
* `nmc_mm256_frexp_pd`

The first three are the equivalent of the corresponding SVML intrinsics without the `nmc` prefix.

The last one is used by the `log` function, but I decided it's nice to have it public.

All results are tested against full precision standard library implementation and provide equal results (except for the `pow` case). `NaN`s and infinities are treated according to the standard.

Limitations:

* Subnormals are treated as zeros by `frexp` and as a result by `log`.