Improve isolation of runtime benchmark compilation

[Kobzol]

This PR improves the "isolation" of runtime benchmarks, in preparation of executing them in CI. This is done in two ways:

1. Benchmark groups (crates) are now compiled in a temporary directory by default, avoiding potential artifact conflicts between different versions of rustc. For compile benchmarks, we copy the whole crate to a temporary directory. I'm not sure if that's needed, so here I just decided to redirect the --target-dir when invoking cargo.

2. Runtime benchmarks are no longer part of the same workspace. While this did speed up their compilation by sharing dependencies, I realized that this could be problematic. Because a dependency change in some runtime benchmark crate (or the addition of a new crate) could change the existing benchmarks, which is something that we want to avoid. The runtime benchmark crates still depend on the benchlib crate, but they do so outside of the main workspace, so any dependency changes will be obvious and reflected in the lockfiles of the individual runtime benchmark crates.

   Sadly, currently due to a limitation of Cargo, every runtime benchmark has to contain an empty [workspace] section (same as for compile benchmarks basically). I removed the exclude section in the last commit, because it was confusing, as it wasn't actually doing anything.

Best reviewed commit by commit. Most changes come from the newly added lockfiles, the actual diff is about 200 lines.