Add TransportFunc and move utilities into under hxutil

[izumin5210]

No description provided.

[codecov-io]

Codecov Report

Merging #12 into master will decrease coverage by 8.85%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
- Coverage    85.9%   77.04%   -8.86%     
==========================================
  Files           8        2       -6     
  Lines         369       61     -308     
==========================================
- Hits          317       47     -270     
+ Misses         32        7      -25     
+ Partials       20        7      -13

Flag	Coverage Δ
#core	?
#pb	77.04% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
option.go
client.go
config.go

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