Allow ChainRules zero types internally

[ToucheSir]

Following on #1385, this allows us to propagate ZeroTangent, NoTangent and NotImplemented inside the boundary of pullback. Doing so should prevent lossy conversions back and forth, as well as inadvertent dropping of the erroring functionality NotImplemented brings. Conversions remain the same for the user-level API, which means this only partially addresses #1227.

This PR can be seen as an incremental step towards #603. There are a couple of options for next steps:

• Allowing Tangents to pass through without conversion
• Supporting thunks (Utilize ChainRulesCore thunks #966)
• Switching all internal rules to use CR zeros instead of Nothing (or converting them to rrules.

PR Checklist

• Tests are added
• [N/A] Documentation, if applicable

[ToucheSir]

Turns out just changing Zygote's own @adjoints to support CR zeros is not enough because downstream package rules are also affected. FluxML/ZygoteRules.jl#25 should help with that and is now a blocker for this PR.