With this change we adjust our rustfmt configuration to include two more
options pertaining the formatting of import/use statements.

With this change we use the Swatinem/rust-cache GitHub action to cache
Rust build artifacts for the build-minimum CI job, which currently takes
the most time to complete. In so doing we more than halve overall CI
wall clock time.

syn::Result allows us to return spanned errors which can be converted
into nice compile errors when there is unexpected input.

For example:
```
error: Failed to parse value, expected a string!
  --> tests/mod.rs:94:33
   |
94 | #[test_log(default_log_filter = 6)]
   |                                 ^
```

Users can now specify a default_log_filter via #[test_log(default_log_filter = "foo")]
which will be used when RUST_LOG is not specified.

Please note that because env_logger / tracing is initialized globally,
it is possible that this value will be ignored if they have already been
initialized by a different test.

Fixes: #25

actions-rs is no longer maintained.

f979a7d bumped MSRV to 1.56.

This should reduce CI time.

.github: replace actions-rs with dtolnay/rust-toolchain, add caching

- Move macros to a new test-log-macros crate.
- Export env_logger and tracing_subscriber from the test-log crate.
- Macros emit test_log::{env_logger,tracing_subscriber} references.

Removes the need for users to explicitly depend on additional crates.

Fixes #36.

Add a CHANGELOG entry for the recent introduction of the test-log-macros
crate.

The 'default_log_filter', while useful, is arguably a bit of a kludge
given that it is inherently racy with respect to other tests
initializing the corresponding log/tracing infrastructure beforehand.
What we would really want is to make it a global attribute, but that is
not currently possible on stable Rust.
Put this attribute behind a newly introduce feature, unstable, so that
we have more flexibility for changing/removing it in the future, without
being required to break semantic versioning guarantees.

This change updates the minimum version of Rust that we support to
1.61.0. We need this version because it is a requirement for env_logger
version 0.10, to which we would like to update subsequently. In the
process we switch the crate over to using Rust edition 2021.

This change bumps the env_logger dependency we use to version 0.10.

Similar to what we did for the test GitHub Action workflow, replace the
deprecated actions-rs/toolchain with dtolnay/rust-toolchain.

This change updates the publish workflow to accommodate for the recent
introduction of the test-log-macros crate to the workspace. Said crate
needs to be published before test-log itself, because test-log depends
on it. In terms of crate versioning we unconditionally release both
crates in lockstep for the sake of simplicity.

This change bumps the version of the crate to 0.2.14. The following
notable changes have been made since 0.2.13:
- Factored out test-log-macros crate to relieve users from having to
  care about tracing/logging dependencies themselves
- Introduced default_log_filter attribute for setting the default log
  filter on a per-test basis behind new unstable feature
- Improved compile error output on wrong usage
- Bumped minimum supported Rust version to 1.61
- Bumped env_logger dependency to 0.10