[WIP] airframe-msgpack: Pure-Scala MessagePack SPI + Implementation

[xerial]

Design considerations:

• pure-scala implementation for the use in Scala (JVM), Scala.js, and Scala.native
• Support offset based packer/unpacker (for better error handling without moving buffer internal cursors)
• Support marking on memory buffers (for holding memory region that contains a value)

[codecov]

Codecov Report

Merging #127 into master will decrease coverage by 83.54%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #127       +/-   ##
=========================================
- Coverage   83.54%     0%   -83.55%     
=========================================
  Files          98     93        -5     
  Lines        3445   3903      +458     
  Branches      321    272       -49     
=========================================
- Hits         2878      0     -2878     
- Misses        567   3903     +3336

Impacted Files	Coverage Δ
...main/scala/wvlet/airframe/codec/MessageCodec.scala	0% <ø> (-66.67%)	⬇️
.../scala/wvlet/airframe/msgpack/io/ArrayBuffer.scala	0% <0%> (ø)
...ala/wvlet/airframe/msgpack/spi/MessageFormat.scala	0% <0%> (ø)
...main/scala/wvlet/airframe/msgpack/spi/Buffer.scala	0% <0%> (ø)
...cala/wvlet/airframe/msgpack/spi/OffsetPacker.scala	0% <0%> (ø)
...ala/wvlet/airframe/msgpack/io/StreamUnpacker.scala	0% <0%> (ø)
...la/wvlet/airframe/msgpack/spi/OffsetUnpacker.scala	0% <0%> (ø)
...in/scala/wvlet/airframe/codec/PrimitiveCodec.scala	0% <0%> (-85.28%)	⬇️
...ain/scala/wvlet/airframe/codec/StandardCodec.scala	0% <0%> (-97.3%)	⬇️
...scala/wvlet/airframe/msgpack/io/StreamPacker.scala	0% <0%> (ø)
... and 114 more

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[xuwei-k]

def packNil: this.type is better?

[xerial]

good point. I'll use this.type