Cargo.toml: Specify optional dependencies with dep: prefix

Although specifying optional dependencies without the dep: prefix works fine for
cargo-level packaging, some OS package managers interpret these as features
rather than optional dependencies. As such, explicitly stating that they are
optional dependencies will make the packaging process easier.

Signed-off-by: Tyler Fanelli <[email protected]>

Cargo.toml

@@ -39,11 +39,11 @@ rustdoc-args = ["--cfg", "docsrs"]
 
 [features]
 default = ["std"]
-bigint = ["num-bigint"]
-bits = ["bitvec"]
-datetime = ["time"]
-debug = ["colored"]
-serialize = ["cookie-factory"]
+bigint = ["dep:num-bigint"]
+bits = ["dep:bitvec"]
+datetime = ["dep:time"]
+debug = ["dep:colored"]
+serialize = ["dep:cookie-factory"]
 std = []
 trace = ["debug"]