gnu: cargo: Simplify unpacking.

Fixes <http://bugs.gnu.org/26166>.

* gnu/packages/rust.scm (cargo)
[arguments]<:modules>: Add (srfi srfi-1).
[arguments]<:phases>: Adapt 'unpack-submodule-sources' phase to more clearly
seperate the tasks it does.  Add helper procedures 'unpack', 'touch',
'install-rust-library'.
[arguments]<:phases>: Rename 'set-cargo-home' to 'set-environment-up' and
make it use official cargo directories.
[arguments]<:phases>: Remove 'configure' phase.