RFR: 7904137: JOL: Improve heap dump parsing speed

[Aleksey Shipilev]

JOL is often used to parse large heap dumps, and it would be good to see if there are simple opportunities in speeding up the heap dump reader. This change lifts up `heapdump-estimates` speed from 800 MB/sec to 1600 MB/sec on my machines.

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Commit messages:
 - Buffer sizes
 - Special path for read_U1
 - Even better skipping
 - More efficient skipping
 - More simplifications
 - Unnecessary alloc
 - Skip reading some useless stuff
 - Improve buffering

Changes: https://git.openjdk.org/jol/pull/68/files
  Webrev: https://webrevs.openjdk.org/?repo=jol&pr=68&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904137
  Stats: 131 lines in 1 file changed: 80 ins; 29 del; 22 mod
  Patch: https://git.openjdk.org/jol/pull/68.diff
  Fetch: git fetch https://git.openjdk.org/jol.git pull/68/head:pull/68

PR: https://git.openjdk.org/jol/pull/68