Individual recovery

[aalavandhan]

• Public method to recover individual assets.
• Cleaned up internal book-keeping of deployed list. Created separate methods to add and remove assets.
• Using <= 0 instead of == 0
• Added some unit tests to improve coverage

Though this helps guard against the _deployed list growing indefinitely. It opens up the surface area of attack if we don't restrict the assets which can be recovered. Example, malicious user can transfer a fake tranche into the system and call recover with the fake tranche address. This adds the tranche into the deployed list. Now the malicious user can fudge the vault's valuation function to drain funds.

We should only allow the user to "recover" vault assets which are already in the deployed list, which we can safely assume to be tranches issued by the whitelisted bond issuer.

[brandoniles]

LGTM