Slepc 3.5.2 (update)

Update SLEPc to the newest version.
Add optional dependency on Arpack.
@davydden How did you use to switch between real and complex versions before you used Homebrew? Isn't that what `PETSC_ARCH` and `SLEPC_ARCH` are for? Perhaps we could have `--real` and `--complex` options to both formulas and they would install either or both scalar versions.