[Documentation] Expand collapsed implementation when clicking on indexed methods

[pravic]

Stable documentation (e.g. https://doc.rust-lang.org/stable/std/collections/hash_map/struct.HashMap.html):

• Click on [-] to collapse everything
• Click on iter in Methods section

Page will be scrolled to the pub fn iter.

Nightly documentation (e.g. https://doc.rust-lang.org/nightly/std/collections/hash_map/struct.HashMap.html):

• Click on [-] to collapse everything
• Click on iter in Methods section

Nothing will happen, because the iter is inside a collapsed impl<K, V, S> HashMap<K, V, S> outer section.

[ehuss]

Another example: Click the word flush in the description on the page https://doc.rust-lang.org/nightly/std/io/struct.BufWriter.html. Nothing happens, making it appear that the link is broken. Since trait impls are collapsed by default, AFAIK there is no way to properly link to them.

I'm guessing this would need to be implemented in hashchange and autoCollapse, and check if the block is hidden?