This updates the libsecp256k1 subtree to the latest master, which includes:
Enabling the GLV endomorphism optimization by default (and removing support for the non-GLV EC multiplication)
Added a proof for the correctness of the lambda split algorithm by roconnor-blockstream (other code was relying on the fact that it always outputs 128 bit results, which isn't at all obvious).
Improved exhaustive tests, in particular for the Schnorr signature module
Various other testing and CI improvements