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]

Codecov Report

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

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

Additional details and impacted files

@@            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	Coverage Δ
src/primitives.jl	49.25% <50.00%> (+0.02%)	⬆️
src/syntax.jl	69.47% <74.41%> (+0.90%)	⬆️
src/ObjectiveC.jl	100.00% <100.00%> (ø)
src/foundation.jl	73.73% <100.00%> (ø)

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