Add support to close cpi accounts, such as TokenAccount

[davoclavo]

Fixes #851

Changes:

• Implement a new trait AccountClose so CPI Accounts can implement a custom close function
• Add an Ours trait marker to be able to tag accounts generated by our program and provide a default AccountClose implementation
• Embed that Ours trait into the codegen for accounts
• Add unit test to be make sure we are able to close a TokenAccount

Shoutout to @cqfd for the assistance!

[Henry-E]

This would definitely be a nice feature to have!

[Henry-E]

This is interesting, did the close constraint ever get support for CPI accounts, like TokenAccount. I have a feeling they did but not sure?