SPL Stake Pool and CLI - v0.6.1

Summary of changes

Program

Add last_epoch_total_lamports and last_epoch_pool_token_supply fields on
the stake pool. These fields are updated every epoch with the previous epoch's
values, to facilitate APR calculations in wallets.

The basic calculation goes:

let pool_token_value = stake_pool.total_lamports as f64 / stake_pool.pool_token_supply as f
64;
let last_epoch_pool_token_value = stake_pool.last_epoch_total_lamports as f64 / stake_pool.
last_epoch_pool_token_supply as f64;

let epoch_rate = pool_token_value / last_epoch_pool_token_value - 1.0;

const SECONDS_PER_EPOCH: f64 = DEFAULT_SLOTS_PER_EPOCH as f64 * DEFAULT_S_PER_SLOT;
const EPOCHS_PER_YEAR: f64 = SECONDS_PER_DAY as f64 * 365.25 / SECONDS_PER_EPOCH;

let yearly_estimated_rate = epoch_rate * EPOCHS_PER_YEAR;