msgpack: add benchmarks

[zchee]

Add benchmarks to the msgpack package.

Update: #94

[codecov]

Codecov Report

Merging #93 (5e3939f) into master (3b7d3b9) will not change coverage.
The diff coverage is 100.0%.

@@ Coverage Diff @@ ## master #93 +/- ## ====================================== Coverage 62.8% 62.8% ====================================== Files 14 14 Lines 2081 2081 ====================================== Hits 1308 1308 Misses 636 636 Partials 137 137 

Flag	Coverage Δ
linux-1.13	62.8% <100.0%> (ø)
linux-1.14	62.8% <100.0%> (ø)
linux-1.15	62.8% <100.0%> (ø)
linux-1.16	62.8% <100.0%> (+<0.1%)	⬆️
macos-1.13	62.8% <100.0%> (+<0.1%)	⬆️
macos-1.14	62.8% <100.0%> (+<0.1%)	⬆️
macos-1.15	62.8% <100.0%> (ø)
macos-1.16	62.8% <100.0%> (ø)
windows-1.13	62.0% <100.0%> (ø)
windows-1.14	62.0% <100.0%> (ø)
windows-1.15	62.0% <100.0%> (ø)
windows-1.16	62.0% <100.0%> (+<0.1%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
msgpack/msgpack.go	66.6% <ø> (ø)
msgpack/pack.go	87.5% <100.0%> (ø)
msgpack/unpack.go	90.6% <100.0%> (ø)

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