Use dep syntax for enabling optional dependencies.

This prevents creating implicit features and also prevents these
from showing up as features that can be enabled when using
`cargo add criterion`.

Author: waywardmonkeys

Cargo.toml

@@ -80,10 +80,10 @@ async = []
 
 # These features enable built-in support for running async benchmarks on each different async
 # runtime.
-async_futures = ["futures/executor", "async"]
-async_smol    = ["smol", "async"]
-async_tokio   = ["tokio", "async"]
-async_std     = ["async-std", "async"]
+async_futures = ["dep:futures", "futures/executor", "async"]
+async_smol    = ["dep:smol", "async"]
+async_tokio   = ["dep:tokio", "async"]
+async_std     = ["dep:async-std", "async"]
 
 # This feature _currently_ does nothing except disable a warning message, but in 0.4.0 it will be
 # required in order to have Criterion.rs generate its own plots (as opposed to using cargo-criterion)
@@ -96,7 +96,7 @@ cargo_bench_support = []
 # This feature _currently_ does nothing, but in 0.4.0 it will be
 # required in order to have Criterion.rs generate CSV files. This feature is deprecated in favor of
 # cargo-criterion's --message-format=json option.
-csv_output = ["csv"]
+csv_output = ["dep:csv"]
 
 [workspace]
 exclude = ["cargo-criterion"]