Upgrade blst to v0.3.12

[jtraglia]

This PR upgrades blst to the latest version (v0.3.12) that was released last month.

[asn-d6]

Can you shortly describe how you do these upgrades? Just so that I can replicate and make sure I get the same results (in particular wondering about the golang headers).

Thanks!

[jtraglia]

Can you shortly describe how you do these upgrades?

Yes, of course. This is what I do:

Upgrade the blst submodule:

cd blst
git fetch
git checkout tags/v0.3.12
cd ..
git add blst

Copy new headers to Go bindings:

cp blst/bindings/blst*.h bindings/go/blst_headers
git add bindings/go/blst_headers

[asn-d6]

Thanks a lot! LGTM!