Use same PrecompileTools.jl approach as REPL to precompile

[IanButterworth]

Without this: (exposing pkg_precompile which it usually isn't)

julia> @time Pkg.pkg_precompile()
  1.796371 seconds (2.23 M allocations: 125.344 MiB, 4.53% gc time, 66.34% compilation time: 9% of which was recompilation)

With

julia> @time Pkg.pkg_precompile()
  0.929547 seconds (555.18 k allocations: 35.390 MiB, 2.02% gc time, 29.93% compilation time: 32% of which was recompilation)

The code is quite small. Do we want to make a utility in Base or just copy it here?

[KristofferC]

Nice nice. For now I think we can just keep it here?