subscriber: feature flags that require std enable it (#1660)

## 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.

tracing-subscriber/Cargo.toml

@@ -26,10 +26,10 @@ keywords = ["logging", "tracing", "metrics", "subscriber"]
 default = ["smallvec", "fmt", "ansi", "tracing-log", "std"]
 alloc = []
 std = ["alloc", "tracing-core/std"]
-env-filter = ["matchers", "regex", "lazy_static", "tracing"]
-fmt = ["registry"]
+env-filter = ["matchers", "regex", "lazy_static", "tracing", "std"]
+fmt = ["registry", "std"]
 ansi = ["fmt", "ansi_term"]
-registry = ["sharded-slab", "thread_local"]
+registry = ["sharded-slab", "thread_local", "std"]
 json = ["tracing-serde", "serde", "serde_json"]
 # Enables support for local time when using the `time` crate timestamp
 # formatters.

tracing-subscriber/src/lib.rs

@@ -50,15 +50,16 @@
 //!   (enabled by default).
 //! - `alloc`: Depend on [`liballoc`] (enabled by "std").
 //! - `env-filter`: Enables the [`EnvFilter`] type, which implements filtering
-//!   similar to the [`env_logger` crate].
+//!   similar to the [`env_logger` crate]. **Requires "std"**.
 //! - `fmt`: Enables the [`fmt`] module, which provides a subscriber
 //!   implementation for printing formatted representations of trace events.
-//!   Enabled by default.
+//!   Enabled by default. **Requires "std"**.
 //! - `ansi`: Enables `fmt` support for ANSI terminal colors. Enabled by
 //!   default.
 //! - `registry`: enables the [`registry`] module. Enabled by default.
+//!   **Requires "std"**.
 //! - `json`: Enables `fmt` support for JSON output. In JSON output, the ANSI
-//!   feature does nothing.
+//!   feature does nothing. **Requires "fmt" and "std"**.
 //! - [`local-time`]: Enables local time formatting when using the [`time`
 //!   crate]'s timestamp formatters with the `fmt` subscriber.
 //!