Misc harness improvements

[frank-emrich]

This PR just collects a few uncontroversial fixes and improvements of the benchmark harness:

• For benchmarks built using make.generic.config, allow specifying additional arguments passed to make as part of the benchmark's definition (useful for defining variants of the same benchmark)
• Use hyperfine's --comand-name option to give useful names to the shell commands it compares. Thus, instead of printing the lengthy shell commands in the Summary table produced by hyperfine, it now shows the benchmark names instead
• Add --prepare-only flag which makes the harness perform all preparation/compilation steps, but stop before actually running benchmarks
• Some minor cleanup about the configuration of CLI arguments
• Actually respect the use-mimalloc CLI flag
• Exit with non-zero exit code if there was an error
• add cargo to the list of helper tools that we look for in $PATH and complain if they are missing

[dhil]

LGTM.