5175: More memory-efficient impl collection r=matklad a=jonas-schievink
This saves roughly 90 MB in `ImplsFromDepsQuery`, which used to copy the list of all impls from libcore into *every* crate in the graph. It also stops collecting inherent impls from dependencies entirely, as those can only be located within the crate defining the self type.
Co-authored-by: Jonas Schievink <jonas.schievink@ferrous-systems.com>
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