Platform definition used by Cargo.
This defines a [`Platform`] type which is used in Cargo to specify a target platform.
There are two kinds, a named target like `x86_64-apple-darwin`, and a "cfg expression"
like `cfg(any(target_os = "macos", target_os = "ios"))`.
See `examples/matches.rs` for an example of how to match against a `Platform`.
[`Platform`]: enum.Platform.html