Feature request: `ExperimentalCycles.addFrom`

[vporton]

Among ExperimentalCycles.add<system> provide also ExperimentalCycles.addFrom<system>

let add : (amount : Nat) -> ()
let addFrom : (subaccount: ?Blob, amount : Nat) -> ();

It should work like add but take cycles from the subaccount.

Also:

let balance : (subaccount: ?Blob) -> (amount : Nat)

Currently I write code that ping-pongs cycles between the subaccount and main account, spending transfer fees in the way. (When it is on subaccount, it can be accounted for a particular user. When on the main account, I can call add<system>) That's not good, that I can't do both.