Add try_to_node which does not panic

[azdavis]

https://github.com/rust-analyzer/rowan/issues/144 hasn't seen any movement and I'm not entirely sure how to fix it myself. So in the meantime it'd be nice to not panic and just fail more gracefully. It's also nice in general as a library to expose the possibility of handling errors, without panicking, to callers.

[azdavis]

@Veykril could this have a look?

[azdavis]

More discussion in the linked issue #144 but the original reason why i opened this pr has been fixed. If we don't think this is useful in general we can close, although it might be nice simply because it lets the caller choose whether they want to panic or not.

As I understand, generally libraries should only panic when:

• Internal invariants are violated (a bug in the library code)
• "Obviously" incorrect arguments are passed

I'm not sure trying and failing to resolve a syntax node pointer fits into either case.

[azdavis]

Could we make a decision either way on this?

[azdavis]

r? @Veykril