Some nofib touchups for recent bytestring/ghc changes and better useability.

Fair number of (mostly) smaller changes, each in it's own commit:

• Add benchmark runtime section to nofib readme
• smallpt - adhere to benchmark duration guidlines for nofib - a benchmark running for a minute is too long
• Added a hackage.head project file. I called it nofib.head. The shake runner will pick it up if one uses --head This can be used via cabal --project-file nofib.head update etc.
• Remove unused reference to source submodule in cabal.project
• A "make it build with head" commit that:
  • Adjusts for the Data.List changes by hiding the generic versions from the Prelude.
  • Adjusts imaginary/integrate for changes in ghc-9 that cause a larger FP error.
  • Adjusts the gc benchmarks. I've mostly added changes that have been added in their non-gc counterparts already.
• Add a verbosity flag to the shake build - The build process used the be extremely noise, now it's just very noisy.
• Added --keep-going to shake, which will ignore stdout/err missmatches.
• Don't run the gc tests by default. Users tend not to care about them. We can explicitly run them in CI if we want.