subscriber: feature flags that require std enable it

[hawkw]

Motivation

Currently, some tracing-subscriber dependencies in other crates use
default-features = false, to ensure that unneeded default features of
tracing-subscriber are not inadvertently enabled. However, some of the
features those crates enable also require the std feature flag. The
default-features = false config disables the std feature, so the
required features are not available.

Solution

This branch changes tracing-subscriber's fmt, registry, and
env-filter feature flags to also enable the std feature flag
automatically.

Alternatives

As an alternative solution, we could change crates that depend on
tracing-subscriber with default-features = false to also explicitly
ensure that the std feature is enabled (which might be worth doing
regardless). This is what PR #1659 does. However, I think the approach
in this branch is more correct; the feature flags that require standard
library support should automatically enable it. This provides a better
experience for most users, who are simply adding default-features = false
in order to avoid enabling un-needed features, not to support
no_std, and would like enabling a feature flag to actually enable
that feature.

no_std users will know not to enable registry, fmt, and
env-filter because the documentation notes that those features require
the standard library.

[hawkw]

This fixes the build failures for tracing-error and tracing-flame.

[davidbarsky]

yeah, this is a good change.

thanks for fixing the flame + error changes; i was too tired to fix them myself yesterday :)