rust: clean remains of direct-ffi code
Since
b55bec1ea972, the cext entry poitn have been removed, so we drop the code
for consistency.
run-tests: add option for running with and without Rust extensions
This provide a simple and clear way to run the test with or without rust.
run-tests: rename "pure" variable to setup_opts
There are more useful flag than just `--pure`. We clarify the variable name
before adding more options.
setup-rust: add a --no-rust flag
This new flag will make sure the rust extension will not be build. If neither
`--rust` nor `--no-rust` is specified the `HGWITHRUSTEXT` is used.
cext: drop the WITH_RUST block
These were used by the `direct-ffi` mode that just got dropped.