[rustdoc] Fix invalid handling of field followed by negated macro call

[GuillaumeGomez]

This is the bug uncovered in #150022. Once fixed Ill rebuild all compiler docs and see if we can enable the option for compiler docs. =D

It's a weird case where we extracted some tokens out of the iterator and then, when checking next items (from this iterator), it didn't find the : token, and therefore badly assumed the token kind.

The solution I came up with is to instead not extract tokens from the iterator and to count how many tokens are in the current path. So when iterate over the items, instead of having a mix of extracted tokens and tokens still inside the iterator, we now only iterate over the iterator.

The biggest change here is that get_full_ident_path will return an option instead of a Vec, and if it's contains : (one, not two), then it will return None and the : will be handled like any token and not like a path (which is more correct imo).

r? @yotamofek

[GuillaumeGomez]

So in case we don't find the item we're interested in, we don't want the item to impact the "peeked" items position, so I needed to write this function to "put back the item" in case it didn't match the condition.

[GuillaumeGomez]

Code doesn't need this continue anymore and that makes it much simpler to understand (so yeah, kinda congratulating myself here ^^').

[GuillaumeGomez]

We don't return in this loop because we need to call stop_peeking before exiting this function. So instead, we break the value to return, call stop_peeking and then actually return. Not super graceful but it works.