Use Cargo.toml custom metadata to specify playground features

[dtolnay]

Is your feature request related to a problem? Please describe.

The Rust playground at https://play.rust-lang.org/ would like to move from using a "playground" feature to using Cargo.toml custom metadata to specify enabled features on the playground. This crate is the last one still using a "playground" feature.

Describe the solution you'd like

Please remove the "playground" feature and specify playground features through custom metadata instead, as already done for docs.rs metadata.

[package.metadata.playground]
features = ["serde", "v1", "v3", "v4", "v5"]

Is it blocking?

The existing support for a "playground" feature will be removed imminently.

Describe alternatives you've considered

You're welcome to weigh in on rust-lang/rust-playground#326.

Additional context

uuid/Cargo.toml

Lines 43 to 46 in 7cb0dac

	# Get features picked up by the Integer 32 playground. Not public API.
	#
	# https://play.rust-lang.org
	playground = ["serde", "v1", "v3", "v4", "v5"]