Delay evaluation of signpost arguments.

[maleadt]

This makes signpost usage really cheap when the logger is disabled:

julia> @benchmark @signpost_event log=OSLog(enabled=false) "disabled event"
BenchmarkTools.Trial: 10000 samples with 1000 evaluations.
 Range (min … max):  3.125 ns … 15.250 ns  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     3.208 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   3.270 ns ±  0.248 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

    ▃ █ ▃ ▂ ▁            ▄ ▂                                 ▁
  ▅▁█▁█▁█▁█▁█▁▇▁▁▇▁▅▁▄▁▆▁█▁█▁▆▁▁█▁▄▁▄▁▄▁▄▁▄▁▅▁▁▃▁▃▁▃▁▄▁▄▁▅▁▃ █
  3.12 ns      Histogram: log(frequency) by time     4.25 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

julia> @benchmark @signpost_interval log=OSLog(enabled=false) "disabled interval" begin end
BenchmarkTools.Trial: 10000 samples with 1000 evaluations.
 Range (min … max):  3.500 ns … 16.000 ns  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     4.667 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   4.816 ns ±  0.721 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

            ▇ ▇  ▆ █  ▃ ▃  ▄ ▆  ▁
  ▁▁▁▁▁▂▃█▆▇█▇██▇██████▆█▆▆█▇█▆▆█▄▆▃▃▄▃▄▂▂▃▂▃▂▂▂▂▂▂▂▃▂▃▁▁▂▁▁ ▃
  3.5 ns         Histogram: frequency by time        6.88 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

Maybe we should do something similar for OSLog, but I don't have a convenient API in mind...

[codecov]

Codecov Report

Attention: Patch coverage is 94.87179% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.23%. Comparing base (a1caf44) to head (965f5a1).

Files	Patch %	Lines
src/os.jl	94.87%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   72.80%   73.23%   +0.43%     
==========================================
  Files          10       10              
  Lines         717      736      +19     
==========================================
+ Hits          522      539      +17     
- Misses        195      197       +2     

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