Fix typos

See merge request !996

maint: replace llvm-tools-preview with llvm-tools

See merge request !993

This commit replaces all occurrences of `llvm-tools-preview` with
`llvm-tools`, because it became a stable feature with Rust 1.60, thereby
making the `-preview` prefix obsolete. Beside this, `llvm-tools-preview`
is a redirection to the latter one anyway.

However, it does not work the other way around. At the moment, it is not
possible to execute the `maint/coverage` script on a NixOS system,
because the rustup component list only includes `llvm-tools`, but not
`llvm-tools-preview`, thereby making this change necessary.

Fix clippy warnings from Rust 1.67

Closes #749 and #748

See merge request !992

Clippy now complains about `let _ = (expr_producing_a_future);`,
which is probably smart, since maybe you wanted to await that future
and ignore the result.  So it seems that the right way to get rid of
an unwanted Receiver is now to drop it explicitly.

Closes #749

In both cases I think it's fine to panic; but it's best to do so
explicitly.

This panics on error, and we're fine with a panic on misbehavior in
tests.

Clippy now warns about these; I'm not sure how these warnings snuck
up on us.

This warning kind of snuck up on us! (See #748)  For now, let's
disable it.  (I've cleaned it up in a couple of examples, since
those are meant to be more idiomatic and user-facing.)

Closes #748.

Update C Tor specific numbers for s101-o3 for January

See merge request !991

Upgrade toml to version 0.6.

See merge request !989

Implement tor-hscrypto

Closes #684 and #742

See merge request !980

tor-netdoc: Provide NetdocEncoder (internal)

See merge request !984

shell: use /usr/bin/env instead of absolute paths

See merge request !988

tor-proto: Mark stream ids in errors as sensitive

See merge request !986

This required rewriting some of our error handling code in
command-line processing, since the toml crate now displays and
reports errors differently.  (Admittedly, this code still is kind of
ugly, but at least it is nicely hidden.)

It turns out that the credential is only calculated as an
intermediate result in order to blind keys and produce the
subcredential. As such, it has no need to leave the hscrypto module.

This commit updates the shellcheck_all script, in order to look for
/usr/bin/env, instead of /bin.

This commit changes the shebang in all shell scripts from absolute
paths (such as `/bin/bash` or `/usr/bin/python3`) to the `/usr/bin/env`
binary with the accompanying interpreter as it's argument.

The reason for this are as follows:
- NixOS cannot work with absolute paths
- BSD systems install their packages in /usr/local/bin

Previously, the offset was set to 12 hours unconditionally (like the
spec says).  But based on a conversation on tor-dev, it seems that
the offset should actually be 12 times the voting interval.

I'm also opening an MR to change the spec.

(I have squashed the fixes for these warnings into the earlier
commits in this branch, so it will look like I did everything right
the first time.)

This implementation was made based on the specification, and then
validated against itself, and against C Tor.

This required some API tweaks, which is probably to be expected;
these time periods are a wonky kind of thing.