Add Mutex::try_lock and (Unbounded)Receiver::try_recv

[bikeshedder]

Motivation

• Mutex::try_lock is missing
• Receiver::try_recv is missing
• UnboundedReceiver::try_recv is missing

This makes it (a) impossible to use tokio mutexes and channels in sync code and (b) implement async code that should try to lock/recv and not block.

Solution

• Add Mutex::try_lock + unit test
• Add Receiver::try_recv + unit test
• Add UnboundedReceiver::try_recv + unit test

Most of the doc strings were taken 1:1 from std and and just adopted to match tokio naming conventions. If this is a problem I can rephrase them in my own words. I think it should be no problem though as the std is dual licensed under MIT and Apache License.

[carllerche]

Looks great! Thanks!