Make crane export more ergonomic

[jonjohnsonjr]

Default second arg to stdout so I don't have to type "-".

For images with a single layer, avoid calling mutate.Extract to flatten
the filesystem, instead we just write the uncompressed bytes to stdout.
In order to make that work, partial.Compressed now actually detects
whether the payload is gzipped or not when calling Uncompressed instead of just
assuming all blobs are compressed (and failing).

Fixes #1150

[codecov-commenter]

Codecov Report

Merging #1203 (3fa962b) into main (c75901c) will decrease coverage by 0.20%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##             main    #1203      +/-   ##
==========================================
- Coverage   74.76%   74.55%   -0.21%     
==========================================
  Files         111      111              
  Lines        8008     8050      +42     
==========================================
+ Hits         5987     6002      +15     
- Misses       1441     1464      +23     
- Partials      580      584       +4     

Impacted Files	Coverage Δ
internal/gzip/zip.go	72.72% <0.00%> (-17.60%)	⬇️
pkg/v1/partial/compressed.go	70.83% <57.14%> (-4.17%)	⬇️
pkg/crane/export.go	68.42% <62.50%> (-31.58%)	⬇️

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[imjasonh]

Nice.