This change allow users to opt out of tracing via the `tracing` crate
by adding tracing as an optional feature. This change is part of the
effort, outlined in hyperium#2874, to reach hyper 1.0.

Closes hyperium#3319
BREAKING CHANGES: tracing is disabled by default and requires users to
opt in to revert to previous behavior.

Introduce several macros that act as wrappers around the `tracing`
module's macros to allow for more concise syntax for conditional
compilation of `tracing` macro calls. As `tracing` is unstable,
the new `trace` module will facilitate transitioning `tracing`
to an optional feature, as outlined in hyperium#2874 and hyperium#3326.

BREAKING CHANGES: Integration of macros requires the replacement of
uses of `trace` macros.

…al trace module

Refactor code to utilize tracing macros from the new `trace` module and
fix breaking changes described in commit aba2a35. Part of the effort to
transition `tracing` to an unstable feature hyperium#3326 #hyperium#2874.

…ompilation.

Usage of variables passed into the `trace` module is dependent on
whether the `tracing` feature is enabled, thus producing a compiler
warning when `tracing` is disabled. Prefix potentially unused variables
with an underscore to indicate that this is intentional.

Add a `rustc` configuration flag (`hyper_unstable_tracing`) that must be passed
to the compiler when using the `tracing` feature. Throw a compiler error if `tracing`
is enabled without the flag. Part of the effort to transtition `tracing` to an
unstable dependecy. See hyperium#3326 and hyperium#2874.

…sage

Add curly braces around a call to the `trace::debug` macro within a
`map_err` anonymous function. The change is necessary to convert the
function from an expression to a block to allow for conditional
compilation.

…ature list

Document `trace` and add new features to the documentation feature list.
Modify `package.metadata.docs.rs` in `Cargo.toml` to include `tracing`
to list of `features` and `--cfg hyper_unstable_tracing` to the list of
`rustdoc-args`.

…arly

Span macros in the `trace` module drop the `Span` early, making them
useless. The fix changes their implementation such that they return the
RAII guard, so the span is consumed and the guard is not dropped.

Remove an extraneous cfg attribute that was added erroneously in a
previous commit.

Remove `trace` documetationfrom `trace.rs`. Add unstable section to
`lib.rs` detailing unstable features. Add information about `tracing`
and `ffi` to unstable features section.

Change condition for when compile error is thrown in the `trace` module,
as the module is compiled even when tracing is disabled. Throw when
`unstable_hyper_tracing` is not specified while tracing flag is enabled.
Removed semi-colons after `Span.entered()` to ensure RAII guard is
returned.

Log errors as a field rather than part of a formatted string.

Similar to ffi, the feature job will skip tracing as it is unstable and
requires a cfg flag to compile without an error. Once hyperium#3326 is merged,
subsequent PRs will fail the feature CI job without this change.