Allow using BLAKE2b checksum instead of bundled MD4

[afbjorklund]

This is mostly for doing performance comparisons, before switching the default checksum.

As discussed, we might want to use more than just 128 bits (out of the available 512 bits).

[afbjorklund]

Currently this is a compile-time flag. It could be made into a runtime option, if wanted (probably not)

[afbjorklund]

As per discussion in #412 this should be made into runtime configuration, so that we can compare.

[afbjorklund]

Made into runtime configuration, but still needs --enable-blake2 to compile.

[afbjorklund]

Now it is back to a compiletime configuration again, removed the checksum setting.

[afbjorklund]

Removed the total counter, and increased the digest from 16 bytes to 20 bytes (out of the 64)

Note that this (digest size) changes the entire checksum, it's not just truncating the full output:

$ echo -n "" | b2sum -l 512
786a02f742015903c6c6fd852552d272912f4740e15847618a86e217f71f5419d25e1031afee585313896444934eb04b903a685b1448b755d56f701afe9be2ce  -
$ echo -n "" | b2sum -l 160
3345524abf6bbe1809449224b5972c41790b6cf2  -

[afbjorklund]

Closing this branch where it is optional, will do another PR where it is simply changed/replaced...