Export compression function

[s1na]

We at EthereumJS are looking into integrating the Blake2b F function into our project (according to this spec).

I understand the compression function is internal to your project, but it'd help us avoid maintaining a fork of this library or re-implementing the function ourselves. I thought I'd create an issue and ask if you're open to the idea of exporting the compression function (with one extra parameter to determine the number of rounds)?

[dcposch]

This is a very cool application of Blake2

I think it's better to keep it separate, though. I took a look at EIP-152, and it looks a bit more general than our unexported blake2bCompress function. It allows a variable number of rounds instead of the standard 12.