implement Base.position

[bicycle1885]

Close #62.

[codecov-io]

Codecov Report

Merging #68 into master will decrease coverage by 10.16%.
The diff coverage is 70%.

@@             Coverage Diff             @@
##           master      #68       +/-   ##
===========================================
- Coverage   87.27%   77.11%   -10.17%     
===========================================
  Files          10       11        +1     
  Lines         393      638      +245     
===========================================
+ Hits          343      492      +149     
- Misses         50      146       +96

Impacted Files	Coverage Δ
src/buffer.jl	95.34% <ø> (-4.66%)	⬇️
src/stream.jl	75.65% <70%> (-5.55%)	⬇️
src/testtools.jl	48.1% <0%> (-51.9%)	⬇️
src/codec.jl	66.66% <0%> (-33.34%)	⬇️
src/transcode.jl	90% <0%> (-10%)	⬇️
src/noop.jl	90% <0%> (-2.86%)	⬇️
src/error.jl	14.28% <0%> (-2.39%)	⬇️
src/io.jl	100% <0%> (ø)	⬆️
... and 2 more

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