1/4 PoS: initial version of the PoS crate

[tzemanovic]

This PR is split from #385. Closes #322

The PoS system and its integration is described in #324. Rendered: https://github.com/anoma/anoma/blob/tomas/pos-design/docs/src/explore/design/pos.md and https://github.com/anoma/anoma/blob/tomas/pos-design/docs/src/explore/design/ledger/pos-integration.md, respectively.

Together with #419, #420, #421, these PRs contain the PoS MVP with:

• validator self-bond
  • tx WASM + client cmd
  • VP validation
• delegation
  • tx WASM + client cmd
  • VP validation
• validator unbond from self-bond
  • tx WASM + client cmd
  • VP validation
• delegator unbond from delegation
  • tx WASM + client cmd
  • VP validation
• validator withdraw from unbond
  • tx WASM + client cmd
  • VP validation
• delegator withdraw from unbond
  • tx WASM + client cmd
  • VP validation
• slashing
  • base ledger apply slash on evidence in BeginBlock
  • withdraw from unbond apply slashes
  • VP validation of slashes on withdraw
• RPC
  • get bonds/unbonds for a given source address
  • get a validator's voting power
  • get total voting power
  • get validator set (active/inactive)
  • add optional arg for epoch for queries above (use current epoch if not specified)

[batconjurer]

A few small comments. I still need to review the validation.rs file

[yito88]

Epoched structs look really nice!
I left comments for lib.rs though I'm reviewing the validation part.

[yito88]

LGTM!