Introduce TraitRefs, remove cell_local (#3887)
`.cell_local`'s semantics are very confusing and can be the cause of
particularly hard-to-debug issues. Furthermore, they can't be nested
correctly, which is necessary for my future HMR work. Instead, @sokra
proposed that we should always use `ReadRef`s instead.
This PR introduces a way to generate `ReadRef`-like `TraitRef`s for a
given trait Vc. The purpose of these refs is only to be passed around,
to be converted back into a `Vc` eventually. You can't call any other
method on a `TraitRef`. This serves the same purpose as `cell_local`, as
you can "snapshot" a value pointed to by a trait Vc through a trait ref,
and re-cell that snapshot at a later time (see the tests).
However, this has the same pitfall as `cell_local`: nested `Vc`s will
still update. @sokra proposed we should have `ValueSnapshot` types which
flatten nested `Vc`s recursively, but this will have to wait until we
have time to tackle tech debt.
This also introduces the `ReadRef::cell` method, which works the same
way as `TraitRef::cell` but can be used on `ReadRef`s instead.
