Add call tracing functionality.

[maleadt]

This should make it easier to file bugs with upstream projects that aren't familiar with Julia:

julia> using ObjectiveC
julia> ObjectiveC.enable_tracing(true)
[ Info: ObjectiveC.jl tracing setting changed; restart your Julia session for this change to take effect!

# restart Julia

julia> using ObjectiveC

julia> str = NSString("test");
+ [NSString stringWithUTF8String: (Int8*)0x000000010dc65428]
  (id<NSString>)0x983d4f92876ccd8c

julia> String(str)
- [(id<NSString>)0x983d4f92876ccd8c UTF8String]
  (Int8*)0x000060000376d6a8
"test"

[codecov[bot]]

Codecov Report

Patch coverage: 75.51% and project coverage change: +0.72 :tada:

Comparison is base (d34df8a) 65.82% compared to head (416acd6) 66.54%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #15 +/- ## ========================================== + Coverage 65.82% 66.54% +0.72% ========================================== Files 8 8 Lines 512 556 +44 ========================================== + Hits 337 370 +33 - Misses 175 186 +11 ``` | [Impacted Files](https://codecov.io/gh/JuliaInterop/ObjectiveC.jl/pull/15?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaInterop) | Coverage Δ | | |---|---|---| | [src/primitives.jl](https://codecov.io/gh/JuliaInterop/ObjectiveC.jl/pull/15?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaInterop#diff-c3JjL3ByaW1pdGl2ZXMuamw=) | `49.25% <50.00%> (+0.02%)` | :arrow_up: | | [src/syntax.jl](https://codecov.io/gh/JuliaInterop/ObjectiveC.jl/pull/15?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaInterop#diff-c3JjL3N5bnRheC5qbA==) | `69.47% <74.41%> (+0.90%)` | :arrow_up: | | [src/ObjectiveC.jl](https://codecov.io/gh/JuliaInterop/ObjectiveC.jl/pull/15?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaInterop#diff-c3JjL09iamVjdGl2ZUMuamw=) | `100.00% <100.00%> (ø)` | | | [src/foundation.jl](https://codecov.io/gh/JuliaInterop/ObjectiveC.jl/pull/15?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaInterop#diff-c3JjL2ZvdW5kYXRpb24uamw=) | `73.73% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaInterop). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaInterop)

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