Add try_into, borrowed_into, and borrowed_try_into attributes

[oblique]

For the borrowed_* the user needs to implement impl<'a> Into<IntoType> for &'a Type. With this the cloing of the value will not happen.

[dtolnay]

Thanks for the PR!

An equivalent attribute was proposed in the past in #1833, and I would prefer not to add it.

[oblique]

@dtolnay what about try_into? I can cherry-pick that in another PR

[dtolnay]

I feel the same about all of them — handwritten Serialize impl is fine for these situations.