This updates the RHEL/ELN rust-toolset macros to the state of
macros.rust and macros.cargo from Fedora rust-packaging v24, with
the following exceptions:
* %__cargo_to_rpm, as cargo2rpm is not shipped in RHEL;
* %cargo_registry, %cargo_instdir, and %cargo_generate_buildrequires
which are for building and/or consuming "library" crates which are not
built in RHEL;
* %__cargo_is_{bin,lib} and the is-lib conditional section of
%cargo_install, as they use cargo2rpm, and only "binary" crates are
built in RHEL;
* Dropped RUSTC_BOOTSTRAP and -Z avoid-dev-deps, as they are overly
broad and should not be needed with vendored builds (rust-packaging#23);
* Dropped rustflags that are already set by default (rust-packaging#24).
