Switch all crates except tools to use Rust 2021

[hudson-ayers]

Edition 2021 enables some optimizations such as disjoint closure captures, along with other niceties such as an expanded prelude. This bring libtock-rs in sync with the kernel, which also uses edition 2021.

So far I just tested that make test and make examples build without warnings, and am relying on CI to catch other issues

[jrvanwhy]

Note that this doesn't switch the workspace to 2021, just the individual crates. E.g. the v1 feature resolver will still be in use.

[hudson-ayers]

Note that this doesn't switch the workspace to 2021, just the individual crates. E.g. the v1 feature resolver will still be in use.

My understanding is that specifying an edition for a workspace is ignored (rust-lang/cargo#5784), and that if we want to switch to using the v2 feature resolver we should put resolver="2" in the workspace. I don't have any feelings about what resolver we should be using, however, so I am inclined to leave it as is for now.

[jrvanwhy]

bors r+

[bors]

Build succeeded:

• ci
• size-diff