add fast sha2 to julia stdlib

[gdkrmr]

@staticfloat, now that the stress of the 1.0 release has passed, please don't forget to add the fast sha2 implementation to the SHA version in Stdlib.

[staticfloat]

@KristofferC what's the best way to get improvements from SHA.jl merged into stdlib?

[KristofferC]

Could do the whole git-external for SHA?

In hindsight, it is a bit silly that SHA is in the stdlib because it is used in one place (https://github.com/JuliaLang/Pkg.jl/blob/87eb9343dd3523d4be23c9c474b9708af3bcb456/src/Types.jl#L42) and only at compile time.

[staticfloat]

And with JuliaLang/julia#41370 we've finally come full circle.