Fix tests on 1.8

[mcabbott]

For #125, this replaces the method definitions for cat functions with slightly less crazy loops, which have fewer ambiguities. In any cat(A,B,Cs...), one of the first two must be a tracked type, no longer one of the first 3, as this shortens the list.

The other failure was depthwiseconv. This is depends on the random number seed, it doesn't consistently fail, I have not investigated whether this is a numerical problem or a gradient problem.

[coveralls]

Pull Request Test Coverage Report for Build 2898925212

• 14 of 17 (82.35%) changed or added relevant lines in 2 files are covered.
• 9 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.7%) to 71.385%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib/array.jl	13	16	81.25%

Files with Coverage Reduction	New Missed Lines	%
src/lib/array.jl	9	66.67%

Totals
Change from base Build 1896746788:	0.7%
Covered Lines:	469
Relevant Lines:	657

---

💛 - Coveralls

[codecov-commenter]

Codecov Report

Merging #128 (f885295) into master (84ff74d) will increase coverage by 0.70%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
+ Coverage   70.67%   71.38%   +0.70%     
==========================================
  Files           9        9              
  Lines         648      657       +9     
==========================================
+ Hits          458      469      +11     
+ Misses        190      188       -2     

Impacted Files	Coverage Δ
src/lib/array.jl	66.66% <81.25%> (+1.05%)	⬆️
src/numeric.jl	100.00% <100.00%> (ø)
src/back.jl	90.38% <0.00%> (+0.09%)	⬆️
src/forward.jl	85.71% <0.00%> (+0.86%)	⬆️

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[ToucheSir]

If you're running tests with >1 threads, that's a long-standing NNlib issue and I'm not sure where it comes from :(

[coveralls]

Pull Request Test Coverage Report for Build 2897172302

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 16 of 19 (84.21%) changed or added relevant lines in 2 files are covered.
• 32 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.7%) to 71.341%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib/array.jl	15	18	83.33%

Files with Coverage Reduction	New Missed Lines	%
src/lib/array.jl	32	66.57%

Totals
Change from base Build 1896746788:	0.7%
Covered Lines:	468
Relevant Lines:	656

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2897172302

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 16 of 19 (84.21%) changed or added relevant lines in 2 files are covered.
• 32 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.7%) to 71.341%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib/array.jl	15	18	83.33%

Files with Coverage Reduction	New Missed Lines	%
src/lib/array.jl	32	66.57%

Totals
Change from base Build 1896746788:	0.7%
Covered Lines:	468
Relevant Lines:	656

---

💛 - Coveralls