Pre-RFC for replacing lib

[epage]

Deviations:

• json output format is different (feat(harness): json output support #26).
  • Should we provide a transition path for unstable users?

[epage]

• eRFC: Custom test frameworks rust-lang/rfcs#2318
  • Tracking issue for eRFC 2318, Custom test frameworks rust-lang/rust#50297
• Tracking issue for #[bench] and benchmarking support rust-lang/rust#29553

[epage]

libs team said that they instead want to shrink the existing libtest in favor of external ones