Use the SHA extension backend with enabled asm feature

[newpavlov]

The SHA extension backend is significantly faster then our ASM code, so with this change on x86 targets we first will check if CPU has the extension and only then we will fallback either to ASM or software implementation.

Closes #220.

cc @dgbo

[not-an-aardvark]

Thanks for making this change. Would it make sense to do the same thing with SHA1 in addition to SHA2? I'm in a similar situation where I'd like to use the SHA1 intrinsic if available, otherwise fall back to the asm implementation. As-is it seems like the asm feature disables the intrinsics.

[not-an-aardvark]

I went ahead and created #225 to apply the same change to SHA1.

[dgbo]

Thank you! This does solve our problem.