WIP: Iterator tracking issues

[vl0w]

Some (smaller / bigger) changes needed to enable iterator support (WIP / tracking PR)

• Moved normalization of constraint param env (for assoc types) for better logging
• Fixed a bug in SpecGraph: Preconditions of the base spec were copied to the constrained spec
• Ghost constraint resolving is a bit relaxed (predicate_may_hold instead of predicate_must_hold_considering_regions) to account for references in associated types. See test associated_types_6.rs
• merge_generics now also merges lifetimes to accout for lifetimes appearing in #[refine_trait_spec]
• Lifetimes in type models are not converted to the anonymous lifetime '_ anymore
• Do not #[derive(Copy, Clone] on type models, because the derive macro "conditionally" implements it when generics are involved. That is, #[derive(Copy)] struct Foo<T> adds impl<T: Copy> Copy for Foo {}, but we do not want to implement Copy only if T is Copy.
• Quantified variables now support associated types: forall(|x: Self::SomeAssocType| ...)

[Aurel300]

bors r+

[bors[bot]]

Build failed (retrying...):

• all-tests (ubuntu-latest)

[bors[bot]]

Canceled.

[Aurel300]

bors r+

[bors[bot]]

Build failed:

• all-tests (ubuntu-latest)

[Aurel300]

bors r+

[bors[bot]]

Build failed:

• all-tests (ubuntu-latest)

[Aurel300]

@vakaras Could you have a look at the two failures here? I think they are the last ones before I can merge this and they are happening only in the unsafe core proof version.

[Aurel300]

bors r+

[bors[bot]]

Build failed:

• test-crates