Adds bootstrap rules to support installing rust-demangler.

When compiling with `-Z instrument-coverage`, the coverage reports are
generated by `llvm-cov`. `llvm-cov` includes a built-in demangler for
C++, and an option to supply an alternate demangler. For Rust, we have
`rust-demangler`, currently used in `rustc` coverage tests.

Fuchsia's toolchain for Rust is built via `./x.py install`. Fuchsia is
adding support for Rust coverage, and we need to include the
`rust-demangler` in the installed `bin` directory.

Configured rust-demangler as an in-tree extended tool.

Added tests to support `./x.py test rust-demangler`.

Install with extended tools by default only if `profiler = true`.

The struct will allow to store more context on the generated tarballs.

Files in the temporary directory are used by ./x.py install.

The option allows to add or remove compression formats used while
producing dist tarballs.

This should not matter right now, but if we ever parallelize rustbuild
this will avoid tarball contents being merged together.

This fixes a bug introduced by #79788.