RFR: 8367324: Avoid redundant parsing when formatting with DigitList

[Johannes Graham]

When formatting doubles or BigDecimals, DigitList first formats them as a string and then parses the resultant string to extract the mantissa and the exponent. This can be done more directly. This allows removing some parsing code and removes a cached byte array.

This also facilitates potential cleanups in FloatingDecimal (removal of getChars method) but I've left that for later to minimize conflicts with other changes there.

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Commit messages:
 - unneeded comment
 - extract fitsIntoLong method, cleanup test
 - Merge branch 'openjdk:master' into digitlist-double4
 - remove unused import
 - add fast path in BigInteger.toString for values that fit into a long
 - add benchmark for BigDecimal formatting
 - avoid parsing

Changes: https://git.openjdk.org/jdk/pull/27118/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27118&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367324
  Stats: 140 lines in 5 files changed: 51 ins; 72 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/27118.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27118/head:pull/27118

PR: https://git.openjdk.org/jdk/pull/27118