Add a view function to display an account's VestingRecord

[isaacdoidge]

An account's VestingRecord cannot currently be accessed via the view methods of 0x1::vesting_without_staking. Consequently, we cannot use the smart contract to calculate the currently claimable amount for a given account. The contract exposes the remaining_grant, but this is the entirety of the remaining allocation, not the unlocked portion.

If we add a view function to display theVestingRecord for an account, then a client can calculate the currently available amount by using the init_amount from the record in conjunction with the start_timestamp_secs, period_duration and schedule of the related VestingSchedule.

We should aim to release this patch in the first framework upgrade.