feat: add depositMany and unassigned convenience functions

[tmigone]

This PR adds a few convenience methods to facilitate deposit management for the gateway/sender:

• depositMany(): allows a sender to make multiple deposits with a single transaction
• depositUnassigned(): allows a third party to deposit escrow amount on behalf of a sender, and more importantly does not assign it to a receiver.
• assignDeposit(): allows a sender to assign unassigned escrow to a receiver
• assignDepositMany(): same as previous but allows assigning multiple amounts to multiple receivers at once.

[ColePBryan]

Looks good, just some small suggestions/ clarifying questions.

[tmigone]

@ColePBryan addressed the comments

[ColePBryan]

Looks good to merge

[tmigone]

We'll wait for audit then merge.