rrule for map

[CarloLucibello]

There used to be a rrule for map, introduced here #56, but I can't find it anymore.

Related to FluxML/Zygote.jl#842

[gxyd]

Using git bisect I can see that it was removed from system in commit: 02e7857

[simeonschaub]

See JuliaDiff/ChainRulesCore.jl#68, we currently lack a proper way for rrules to call back into the AD system, so ChainRules can't really deal with higher-order functions yet.

[nickrobinson251]

Xref #222

[oxinabox]

#222 is the version of this that we can without doing JuliaDiff/ChainRulesCore.jl#68 first.

[mzgubic]

No longer blocked as of JuliaDiff/ChainRulesCore.jl#363