Pass full feature list in environment variable

[rmccue]

Problem

Cargo passes over features one-by-one in CARGO_FEATURE_... environment variables:

cargo/src/cargo/core/compiler/custom_build.rs

Lines 224 to 228 in 524231f

	// Be sure to pass along all enabled features for this package, this is the
	// last piece of statically known information that we have.
	for feat in &unit.features {
	cmd.env(&format!("CARGO_FEATURE_{}", super::envify(feat)), "1");
	}

However, there's no way to get a list of all of these variables, without iterating over every environment variable.

I'd like to be able to display the list of enabled features in the version information, and right now that requires testing each variable (via #[cfg(feature=)]).

Proposed Solution

A new environment variable ala CARGO_FEATURES with a comma-separated list of enabled features would allow grabbing this information easily. (Comma-separated would match the --features flag to cargo)

Notes

One potential downside of this is that it adds redundant information, and might make it a bit worse for users invoking the toolchain directly without using the cargo tool specifically.