implement BLS signatures

[phritz]

Placeholder for actual work.

Additional acceptance criteria:

• maybe close out Signature code and tests should be in one, reasonable place. #1326?
• new code has unit tests
• no new shortcuts taken in tests -- tests work at least as "real" as they do today unless we agree otherwise
• talk to go-filecoin team about what this change means before it is implemented in order to get feedback, eg at dev-wide meeting, in an issue, or at storytime; also consider an implementation plan sketch to reduce risk
• frrist timeboxed to <=20% of his time on average (can be more during ramp up)
• this work is done done when this issue is closed out, meaning zero technical debt deferred, follow-up issues, TODOs for missing features that could be implemented today, tests, places in the code where we should be using it but aren't, etc. ; see "do not create technical debt" section of https://github.com/filecoin-project/go-filecoin/blob/master/CONTRIBUTING.md#developer-dos-and-donts

[frrist]

closing in favor of #1606, I have moved this issue description to the epic.