gnu: rust: Update rust to 1.24.1.

gnu: rust: Update rust to 1.24.1.

* gnu/packages/rust.scm (rust-bootstrap): Freeze bootstrap version to 1.22.1.
Reorganize bootstrap binaries to match new "rust" package structure with two
ouputs "out" with Rust compiler and "cargo" with Cargo package manager.
Replace all "system*" by "invoke".
(rust-1.23): Rename "rustc" to "rust".  Update sha256 for tarball.  Add cargo
related dependencies.  Install "cargo" as separate "rust" output.  Remove
substitutes for parts that were fixed in "rustc" source code.  Install
documentation to separate "doc" output.  Update configuration according to
changes in "rustc" source code.  Replace all "system*" by "invoke".
(rust): Create package for 1.24.1 rust release based on rust-1.23 release.
* guix/build-system/cargo.scm (default-rust): Replace "default-cargo" and
"default-rustc" functions with "default-rust" function.
(cargo-build-system): Take only one package as "rust" source.  Use "rustc" of
default output of "rust" package, use "cargo" as "cargo" output of "rust"
package.