bumps constantine, updates curves

[benbierens]

Is needed to fix broken build of multihash-poseidon2 branch over in nim-libp2p.

[2-towns]

Updated to the last version of Constantine because of a compilation error with Nim 2.X.

I added a nimble.lock file. You can use it by adding the env variable:

NIMBLE_DIR=.nimble
NIMBLE_DIR=.nimble nimble install

Or by passing the command arguments:

nimble c --nimbleDir=.nimble

[2-towns]

Note that Constantine removed the support to the Nim version 1.6.x

[mratsim]

Note that I dropped explicitly testing in CI the 1.6.x series but it should still compile.

The reason why is that I couldn't compile this Ethereum Foundation funded PR to accelerate Secret Leader Election: mratsim/constantine#485. But currently it's not used anywhere.