Prepare Cargo for `resolver = "2"` as the default in the 2021 edition

[alexcrichton]

In the 2021 edition for Cargo we're planning on having resolver = "2" be the default for all projects (e.g. if omitted then resolver = "2" will be inferred if edition = "2021"). Some work items that need to be implemented for this are:

• Execution of cargo fix --edition where resolver is not specified needs to generate a warning if the 1/2 feature graphs differ. Ideally we'd also print differences here, but at the very least Cargo needs to recommend resolver = "1" be written down to avoid breakage in 2021.
• We need to default to resolver = "2" if not otherwise specified in the 2021 edition.
• Workspaces may need an edition = "..." specifier.
• Documentation for the edition will need to be prepared about this change in Cargo, probably referencing the newly-rewritten features chapter.