Add a struct HashMapRef which holds a reference and guard

[cuviper]

This is created by HashMap::as_ref(&self), and it implements most of
the same public API, just with implicit epoch guards. Additionally, it
implements IntoIterator by reference, and a new trick -- Index!

[cuviper]

it implements most of the same public API

Obvious absences are all of the HashMap constructors. In addition, I didn't add retain and retain_force because those take &mut self instead of &self -- was that intentional? They talk about concurrent access, but &mut prevents that entirely.

I also need to add tests, but I wanted to float the idea in a PR before I continue.

[codecov]

Codecov Report

Merging #45 into master will increase coverage by 2.19%.
The diff coverage is 71.18%.

Impacted Files	Coverage Δ
src/map.rs	87.34% <100%> (+1.13%)	⬆️
src/map_ref.rs	66.66% <66.66%> (ø)
src/raw/mod.rs	93.02% <0%> (+6.97%)	⬆️
src/node.rs	94.5% <0%> (+49.05%)	⬆️

[jonhoo]

Ah, that is just beautiful!

No, &mut self for retain was not intentional. Feel free to fix in this PR.

[jonhoo]

I pushed a fix for retain taking &mut to master.

Also, #44 just added explicit guards to a bunch more methods, which may be a good precursor to the work here.

[cuviper]

Hmm, maybe I should just cross-link docs, instead of duplicating them...

[jonhoo]

Yeah, I think that's probably fine, given that they are "feed-through" anyway. I wish there was a way to #[doc(include(path::to::type::or::method))].

[jonhoo]

Oh, also, you may want to rebase on/merge master given that I merged #47 which included a bunch more guarded methods.

[jonhoo]

Fantastic, thank you!