Implement v1 image tarball generation in new legacy package

[smukherj1]

Fixes #526
Add functions that can generate docker image tarballs in the v1 format. This is what docker save generates by default in version 18.09.3 today.

This is the same as #535 but adds the code to a new legacy package instead of the v1 package.

[codecov-io]

Codecov Report

Merging #536 into master will decrease coverage by 0.61%.
The diff coverage is 62.27%.

@@            Coverage Diff             @@
##           master     #536      +/-   ##
==========================================
- Coverage   73.25%   72.63%   -0.62%     
==========================================
  Files          94       95       +1     
  Lines        4015     4221     +206     
==========================================
+ Hits         2941     3066     +125     
- Misses        703      759      +56     
- Partials      371      396      +25

Impacted Files	Coverage Δ
pkg/v1/partial/with.go	64.03% <ø> (ø)	⬆️
pkg/v1/partial/uncompressed.go	46.42% <0%> (-10.1%)	⬇️
pkg/v1/tarball/write.go	60.82% <100%> (ø)	⬆️
pkg/v1/tarball/image.go	72.88% <36.84%> (-4.37%)	⬇️
pkg/legacy/tarball/write.go	71.02% <71.02%> (ø)

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[jonjohnsonjr]

Can you add a pkg/legacy/doc.go and pkg/legacy/tarball/doc.go with package comments?

[jonjohnsonjr]

This is huge, thanks :)