experimental plonk support and user documentation

hotfix to v0.3.7: changes on linear expressions caused some circuit i…

…nit to fail

minor fix to v0.3.6: less memory hungry tests for circleCI with short…

… mode on

serialization, gurvy and pairing improvments, minimized R1CS constrai…

…nts with Add or Sub operations

v0.3.4: fixes a data race in the multiexp