Add an adapter interface.

[Maikuolan]

When the intention exists to write numerous classes with the same or very similar methods, PHP interfaces are a useful means to ensure that accidents don't occur with method names in the future, and to ensure that such intentions are fully met insofar as such classes possessing the needed methods.

This pull request adds an adapter interface (this contributes towards the work currently being done at the dev@220 branch). This pull request also slightly cleans up some of the PHPDoc comments, and adds both parameter type hints and return types to all methods which belong to the affected classes.

[codecov[bot]]

Codecov Report

Merging #321 (93f0611) into dev@220 (c547cf6) will not change coverage. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             dev@220     #321   +/-   ##
==========================================
  Coverage      10.10%   10.10%           
  Complexity      1947     1947           
==========================================
  Files            114      114           
  Lines           5183     5183           
==========================================
  Hits             524      524           
  Misses          4659     4659           

Impacted Files	Coverage Δ	Complexity Δ
src/Archive/Adapter/Bzip.php	0.00% <0.00%> (ø)	12.00 <12.00> (ø)
src/Archive/Adapter/Gzip.php	0.00% <0.00%> (ø)	12.00 <12.00> (ø)
src/Archive/Adapter/Zip.php	0.00% <0.00%> (ø)	13.00 <13.00> (ø)
src/Archive/Archive.php	0.00% <0.00%> (ø)	7.00 <2.00> (ø)

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[peter279k]

• I wonder why this PR is compared to the dev@220 branch rather than master branch.
• All latest commits are available on master branch, not on dev@220 branch.

[Maikuolan]

• I wonder why this PR is compared to the dev@220 branch rather than master branch.

This is intentional. This PR proposes some changes to the archive files, which currently exist in dev@220, but which don't yet exist in master (due to still being worked on at #311).

My idea behind targeting dev@220 is that, if and when this PR is merged, the commits here (#321) will appear at the end of the list of commits at the other PR (#311), but not yet at master (since the archive files are still being worked on). Then everything can appear in master later, after we're ready to merge the other PR. :-)