Avoid capturing external variables into closures

[CarloLucibello]

This PR works around a notorious performance problem with closures in julia
JuliaLang/julia#15276
that recently triggered a bug in Zygote
FluxML/Zygote.jl#1290
that made NNConv gradient tests fail (#208)

We now avoid message function capturing external variables by adding additional propagate and apply_edges method.
These new methods take as input a GNNLayer as well and pass it to the message function.

We also make all internal types concrete for conv structs, which could improve performances on small graphs.

Fix #208

[codecov]

Codecov Report

Merging #225 (55e6ed8) into master (45a14db) will decrease coverage by 0.73%.
The diff coverage is 82.71%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
- Coverage   87.31%   86.58%   -0.74%     
==========================================
  Files          15       15              
  Lines        1411     1431      +20     
==========================================
+ Hits         1232     1239       +7     
- Misses        179      192      +13     

Impacted Files	Coverage Δ
src/msgpass.jl	71.42% <66.66%> (-3.58%)	⬇️
src/layers/conv.jl	76.78% <85.50%> (-1.39%)	⬇️

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