Update rust-toolchain version

[remoun]

Fixes #1408. This includes a newer version of Clippy, so fix those findings, one per commit.

I tried updating to a more recent nightly, but ran into an unknown issue consistently causing Clippy to crash; I filed rust-lang/rust-clippy#8527.

[iamalwaysuncomfortable]

Looks good, I think most of the #dead code annotations are on variables meant to preserve lifetimes (i.e. the lmdb envs) can be removed with the use of underscore naming.

[iamalwaysuncomfortable]

LGTM!

[jcape]

Generally LGTM, but the couple places in mobilecoind where we are grabbing an LMDB environment handle and hanging on to it but never using it, except in tests can be better represented.

It looks a bit ugly, but IMO the situation is a big ugly ;-)

[cbeck88]

I'm unsure about removing the Arc<Lmdb:: Environment>. I think that there is some unsoundness in the Lmdb crate and many of the Lmdb::Database objects will crash if all the Environment are collected. I think it might be better to just put _env: ... to silence clippy.

I think the changes in SGX crates look good, we should make some kind of follow up ticket for removing features like panic_unwind, backtraces, since I don't think that stuff is going to happen, and it would let us simplify the code

Generally looks good to me, thank you!

[iamalwaysuncomfortable]

Had a nit on the must_use annotations but the context where they are is clear enough. These changed look good to go @garbageslam what say you - this ready to go?

[iamalwaysuncomfortable]

Good thoughtful discussion on subtle yet important paradigms in the MC code here, thanks @remoun for bringing these to the forefront and thanks all for the discussion.

[cbeck88]

LGTM