Staking support on behalf of another address

[kautukkundan]

What does this pull request do? Explain your changes. (required)

Modified bonding manager contract to extend "stake for" feature. This allows an address to stake LPT on behalf of another address.

Specific updates (required)

• modified bondWithHint function to accept onBehalfOf address
• modified bond function to pass msg.sender as onBehalfOf
• added a new function bondOnBehalf

How did you test each of these updates (required)

Does this pull request close any open issues?

closes #453

Checklist:

• [ ] README and other documentation updated
• [ ] All tests using npm run test pass

[kyriediculous]

Cool that you opened a draft PR for this, I have some ideas that can be useful to improve the API a bit that I didn't consider before.

My current thinking is that we can remove the functions that don't accept hints and just have those that do. If hints then become the default we can just drop the withHint suffix.

Then I think it would be good to separate the two functions:

• bond(uint256 _amount, address _orchestrator, ... hints) which calls the internal function and supplies msg.sender as _for argument
• bondFor(uint256 _amount, address _orchestrator, address _for, ... hints) which calls the internal function with the same arguments as provided in the external call

and an internal function

• bond(uint256 _amount, address _orchestrator, address _for, ... hints)

[kautukkundan]

Added some basic tests for bonding manager bond function.