Allow saving of 'recursive' immutable structs

[JonasIsensee]

When serializing immutable structs, JLD2 attempts to generate an in-line representation for the given struct.
It does so by recursively trying to find the memory layout of all immutable fields.
This is neat for many applications but it fails when encountering recursive structs e.g.

struct RecursiveStruct
    x::RecursiveStruct
    RecursiveStruct() = new()
    RecursiveStruct(x) = new(x)
end

This PR modifies the relevant function hasfielddata and hasdata to look for such recursive structs.
The functions now track which types they already have encountered and terminate the inlineing
when a type signature is encountered again.

closes #189

[giordano]

I can't comment on the implementation as I'm not familiar with the code of the package, but I tried this PR with Measurements.jl and it seems to work great:

julia> using JLD2, Measurements

julia> x = 3 ± 1
3.0 ± 1.0

julia> y = 2x
6.0 ± 2.0

julia> y - x - x
0.0 ± 0.0

julia> z = 6 ± 2
6.0 ± 2.0

julia> z - x - x
0.0 ± 2.8

julia> @save "output.jld2" x y z

julia> JLD2.jldopen("output.jld2") do io
           io["y"] - io["x"] - io["x"]
       end
0.0 ± 0.0

julia> JLD2.jldopen("output.jld2") do io
           io["z"] - io["x"] - io["x"]
       end
0.0 ± 2.8

{Un,}correlated variables are handled corrected when deserialised, too!

[oschulz]

Thanks for this!

[oschulz]

Would it be possible to have a new release of JLD2 that includes this? This functionality would be very helpful for us.

[JonasIsensee]

Hi,
I'm trying to prepare a somewhat bigger release.
In particular I would like to include #206 but I am not super happy with that PR just yet.

I'd say the release is no more than 2 weeks away. If I can't fix #206 until then, I'll put out a release without it.
Does that sound alright for you?

[oschulz]

Thanks, that sounds great! Two weeks is not a problem.