add a small precompile workload

[KristofferC]

Using

@time @eval Downloads.download("https://pkg.julialang.org/registries", tempname())

as a test (because that is the end function I am interested in) we get on master:

0.838721 seconds (489.98 k allocations: 34.457 MiB, 41.40% compilation time)

and with this PR:

0.484503 seconds (31.97 k allocations: 2.184 MiB, 4.42% compilation time)

[codecov]

Codecov Report

Merging #226 (a840de1) into master (3ed0f08) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #226   +/-   ##
=======================================
  Coverage   90.26%   90.26%           
=======================================
  Files           5        5           
  Lines         575      575           
=======================================
  Hits          519      519           
  Misses         56       56           

Impacted Files	Coverage Δ
src/Downloads.jl	85.58% <ø> (ø)

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[StefanKarpinski]

Does an extra half a second on the first download really matter?

[KristofferC]

Yes. 0.5s * many things becomes many seconds. This is one small piece of a larger puzzle.