Various cleanup

[taiki-e]

• Add cargo llvm-cov report subcommand. This is almost the same as cargo llvm-cov --no-run, but it has a more obvious name and better diagnostics.
• Add cargo llvm-cov test subcommand. This is equivalent to cargo llvm-cov without subcommand, except that test name filtering is supported.
• Deprecate --no-run in favor of cargo llvm-cov report subcommand.
• Support --keep-going (unstable), --ignore-rust-version.
• Support --exclude-from-report and --ignore-run-fail for cargo llvm-cov run.
• Fix help messages for subcommands.
• Fix handling of nextest's --profile option.

Closes #92
Fixes #216

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci