Check object lifetime bounds in coercions, not just trait bounds.

[nikomatsakis]

Check object lifetime bounds in coercions, not just trait bounds. Fixes #18055.

r? @pcwalton

This is a [breaking change]. Change code like this:

fn foo(v: &[u8]) -> Box<Clone+'static> { ... }

to make the lifetimes agree:

// either...
fn foo(v: &'static[u8]) -> Box<Clone+'static> { box v }

// or ...
fn foo<'a>(v: &'a [u8]) -> Box<Clone+'a> { box v }