Add function for parsing already-read manifest
Add function for parsing already-read manifest

This makes the function formerly known as `do_read_manifest` public
under the name `read_manifest_from_str` so that Cargo-as-a-library users
can parse a manifest without re-reading a `Cargo.toml` file they have
already read from disk themselves.

rustyhorde/cargo,rustyhorde/cargo,nipunn1313/cargo,rustyhorde/cargo,eminence/cargo,eminence/cargo,nipunn1313/cargo,rust-lang/cargo,eminence/cargo,rust-lang/cargo,nipunn1313/cargo,rust-lang/cargo,rust-lang/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo