std: Deprecate RefCell::{try_borrow, try_borrow_mut}

[alexcrichton]

The existence of these two functions is at odds with our current error
conventions which recommend that panicking and Result-like
variants should not be provided together.

This commit adds a new borrow_state function returning a BorrowState enum to
RefCell which serves as a replacemnt for the try_borrow and try_borrow_mut
functions.

[rust-highfive]

r? @aturon

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[aturon]

@bors: r+ deed093

[tbu-]

This makes it a lot more unergonic to use and is not mentioned in the linked-to RFC. In the RFC it says that RefCell and Channel are the current exceptions.

[alexcrichton]

@tbu- we've been backpedaling on there being any exceptions to the rule by moving channel methods entirely to Result and this was the final nail in the coffin (to stop having RefCell be the only exception).

[bluss]

I'm sad, it makes RefCell even more cumbersome to use, and it's for example harder to provide a custom panic message. I'd opt for just using .borrow_mut() instead now sadly. Using Option would feel more rustic.

This is not like channel because there is no option- or result-returning method left.