Problems with the `DEFINED_DIFFRULES` implementation

[KristofferC]

Many "introspection" functions in DiffRules like DiffRules.diffrules look at the global variable DEFINED_DIFFRULES to collect information about what diff rules are defined. I think this has some issues:

• Only diff rules defined inside DiffRules.jl will be inside DEFINED_DIFFRULES. Packages that use @define_diffule will not be in that list. That is because side effects like adding to a global variable are not visible when modified during the time a package gets precompiled. You would have to add stuff to it in __init__ for it to be visible.
• Accesses are made with a symbol as an argument, for example, DiffRules.hasdiffrule(:Base, :sin, 1). This feels to me like it should be made with a module as first argument.

I encounter this when I try to make e.g. LogExpFunctions into an extension (which makes it a separate module). I'm trying to think of ways to improve this that are backwards compatible.