RFR: 6651: Exposing the magic bytes for the compressed formats supported

[Marcus Hirt]

Sometimes you have a stream from elsewhere, and need to do some custom magic.

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Commits:
 - 974055b9: 6651: Exposing the magic bytes for compressed formats supported

Changes: https://git.openjdk.java.net/jmc/pull/13/files
 Webrev: https://webrevs.openjdk.java.net/jmc/13/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6651
  Stats: 21 lines in 1 file changed: 7 ins; 0 del; 14 mod
  Patch: https://git.openjdk.java.net/jmc/pull/13.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/13/head:pull/13

PR: https://git.openjdk.java.net/jmc/pull/13