some explicit precompile statements and precompile-friendly loadtime hooks

[ranocha]

On my system, I get the following results.

Current master:

$ julia --project=. -e 'import Pkg; Pkg.precompile(); @time @eval using MPI'
Precompiling project...
  1 dependency successfully precompiled in 3 seconds. 13 already precompiled.
  0.068064 seconds (60.26 k allocations: 4.630 MiB, 51.31% compilation time)

This PR:

$ julia --project=. -e 'import Pkg; Pkg.precompile(); @time @eval using MPI'
Precompiling project...
  1 dependency successfully precompiled in 3 seconds. 13 already precompiled.
  0.038507 seconds (57.11 k allocations: 4.375 MiB, 6.25% compilation time)

[ranocha]

Bump

[ranocha]

Thanks for the review and for merging this, @giordano. It would be nice to get a new release of MPI.jl 🙂