Rename internal_transfer -> internal_transfer_locked
The new instruction can move only unlocked funds and is useful to avoid
needing to withdraw funds if they should be re-locked in a different
deposit entry.
Withdrawing can be impossible when a voter is engaged in proposals.
Adds LockupKind::Constant, extends the reset_lockup instruction and
adds the internal_transfer instruction to allow working with constant
maturity lockups.
Christian Kamm