use static as object-lifetime default for type XX in `Foo<Item=XX>`

use static as object-lifetime default for type XX in `Foo<Item=XX>`

Currently the default is "inherited" from context, so e.g.  `&impl
Foo<Item = dyn Bar>` would default to `&'x impl Foo<Item = dyn Bar +
'x>`, but this triggers an ICE and is not very consistent.

This patch doesn't implement what I expect would be the correct
semantics, because those are likely too complex. Instead, it handles
what I'd expect to be the common case -- where the trait has no
lifetime parameters.