Slepc 3.5.2 (update)

Update SLEPc to the newest version.
Add optional dependency on Arpack.
The supported way to accomplish this would be to have separate formulae and switch between them manually (or using a script or something) or make them keg-only and use the opt paths to reference them.

This is clever, but having the formula version depend on other installed packages makes me really uneasy. The long-term goal is to make the DSL fully declarative, i.e. devoid of conditionals or code that "performs work".