Add incentives to proof-of-stake

[asaj]

Description

This PR adds incentives to proof-of-stake, specifically epoch payments for validators and groups, and epoch rewards for voters.

Tested

Unit and end-to-end tests.

Other changes

• Move end-to-end governance test to contract kit
• Factor out fractionMulExp into UsingPrecompiles.sol
• Automatically mark groups as eligible for election when adding the first member
• Remove minter from StableToken, and use the registry instead
• Store historical authorized Validator and Voter information
• Point to ganache with getEpochSize precompile

Related issues

TODO

• Fixes #[issue number here]

Backwards compatibility

Not backwards compatible