Fix anymutable errors with self-referencing types

[oschulz]

I currently have trouble using fmap with data structures that contains Measurements.Measurement objects. The reason is that Measurement (which subtypes Real) contains a field Derivatives:

struct Derivatives{T} <: AbstractDict{Tuple{T,T,UInt64},T}
    parent::Derivatives{T}
    ...
end

Due to the nature of the type, this parent field may be undefined - quite normal for such datatypes.(Measurements.Derivatives is a variation of Base.ImmutableDict, apparently).

This causes Functors.anymutable to fail, due to

@generated function anymutable(x::T) where {T}
  ...
  subs =  [:(anymutable(getfield(x, $f))) for f in QuoteNode.(fieldnames(T))]
  ...
end

While I have this problem with Measurement specifically, I think it makes sense to fix it here, instead of proposing a Functors extension to Measurements. Also, as expected (see above) the same problem occurs with Base.ImmutableDict:

julia> fmap(identity, Base.ImmutableDict(:b => 42))
ERROR: UndefRefError: access to undefined reference

A simple fix would be checking with isdefined (if the field is not defined, it obviously doesn't point to mutable content):

@generated function anymutable(x::T) where {T}
  ...
  subs =  [:(isdefined(x, $f) && anymutable(getfield(x, $f))) for f in QuoteNode.(fieldnames(T))]
  ...
end

I worry that this may reduce the type stability of anymutable though. So in this PR I'm proposing

@generated function anymutable(x::T) where {T}
  ...
  fns = QuoteNode.(filter(n -> fieldtype(T, n) != T, fieldnames(T)))
  subs =  [:(anymutable(getfield(x, $f))) for f in fns]
  ...
end

In the end, mutability is a type property, I'd say: if T itself is not directly mutable, and all of it's fields except references to T are immutable, then I think it's safe to consider x immutable as a whole.

[oschulz]

Also fixes the broken docs generation (was not Documenter v1 compatible, but Documenter version bound was missing in "docs/Project.toml"). I updated everything to Documenter v1 now.

[oschulz]

@ToucheSir, if you have time, could you take a look at this?

[ToucheSir]

I didn't quite understand what this was doing yesterday, but with a fresh set of eyes it makes sense now :)

[oschulz]

Whoops, sorry @ToucheSir , I had overlooked the fact that the version number had already been increased, so JuliaRegistries/General#100422 is currently stalled.

Could you call Registrator on commit 19788ea so that we can get v0.4.6 registered first?