Fixes infinite recursion when we hit a specific case of an itertools
iterator, a few visual glitches (too many or few line breaks), and adds
the struct type distinction.

Some deficiencies mean we don't currently get all information. For
example, qualifiers such as extern ABI and unsafe are dropped from
methods and the information has_body is true even for provided methods.

Trait bounds and generics are still missing. Unifies the way that impls
are scheduled such that we might group them soon.

They appear as ExternCrateItem 😂

That is obviously wrong and in particular this loses any information on
the factual item name, on the crate origin, the path, and possibly more.
We nevertheless at least try to render them here which works decently
except for globs.

Also gates header levels to those that the backend actually supports.
There is no subsubsubsubsection.

We had to make one slight adjustment in the order of Union and Struct,
since they appear to have the same items now. This means that union
can match union, with the type as an extra field that is ignored!

Uses the header adjustment strategy previously introduced to the code to
ensure that scoping is correct. Most doc comments contain an h1 instead
of the proper header level from the context. Note that in particular
this will put some pressure on the backend to allow for slightly deeper
nested headers, at least to 6. We already emulate them by a weighted
font in one case.

It's important that this is the first step. This avoid any potential
error that would come from trying to serialize a field that has an
incompatible type, which would lead to the incorrect diagnosis that rust
has output broken documentation.