Track requirements for Rust nightly

[daviddrysdale]

We currently use a fairly recent nightly build of Rust; it would be helpful to track the reasons why we need to do this, e.g.:

• Support for ThreadSanitizer runs without false positives (Set up thread-sanitizer run of Rust runtime in CI #778)

[daviddrysdale]

It looks like we have two things that require nightly channel (see #1036):

• Running ThreadSanitizer is blocked on Tracking issue for sanitizer support rust-lang/rust#39699
• The rustfmt example, which has been disabled since Update Rust version #588 (Feb 2020) anyway (see Reenable rustfmt example when rustc ICE is fixed #594). (Removed in examples: drop rustfmt example #1076)
• The stable cargo-doc doesn't cope with inter-crate links well, and emits some invalid links (blocked on Tracking issue for RFC 1946 - intra-rustdoc links rust-lang/rust#43466). (Appears to no longer be the case with current codebase layout and 1.44.0)

The former looks unlikely to be resolved soon.

[daviddrysdale]

Current things that require nightly channel (see #1036):

• Running ThreadSanitizer is blocked on Tracking issue for sanitizer support rust-lang/rust#39699. We could get around this by specifically using nightly Rust for scripts/run_tests_tsan (as in Shift to using stable Rust #1036).
• Our rustfmt config requires features that are only available in nightly Rust (comment_width, format_code_in_doc_comments, imports_layout, merge_imports, normalize_doc_attributes, wrap_comments).

[daviddrysdale]

Also (since #1230):

• Specifying the output location for built binaries (--out_dir) requires -Z unstable-options (blocked on --artifact-dir (nee --out-dir) Tracking Issue rust-lang/cargo#6790)