RFR: 8315454: Add an immutable BitSet [v2]

[Per Minborg]

> This PR proposes adding a new method to BitSet that provides an immutable snapshot of the set in the form of an `IntPredicate`.
> 
> The predicate is eligible for constant folding.
> 
> Here are some classes in the JDK that would benefit directly from constant-folding of BitSets:
> 
> PoolReader (6)
> URLEncoder (1) - Updated in this PR
> HtmlTree (2)
> 
> More over, the implementation of the predicate is @ValueBased and this would provide additional benefits in the future.
> 
> Initial benchmarks with the URLEncoder show encouraging results:
> 
> 
> Name                           (encodeChars) (maxLength) (unchanged) Cnt  Base   Error   Test   Error  Unit  Diff%
> URLEncodeDecode.testEncodeUTF8             6        1024           0  15 2,371 ± 0,016  2,073 ± 0,184 ms/op  12,6% (p = 0,000*)
> URLEncodeDecode.testEncodeUTF8             6        1024          75  15 1,772 ± 0,013  1,387 ± 0,032 ms/op  21,8% (p = 0,000*)
> URLEncodeDecode.testEncodeUTF8             6        1024         100  15 1,230 ± 0,009  1,140 ± 0,011 ms/op   7,3% (p = 0,000*)

Per Minborg has updated the pull request incrementally with three additional commits since the last revision:

 - Fix test
 - Remove unused imports
 - Convert into an internal API

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15530/files
  - new: https://git.openjdk.org/jdk/pull/15530/files/e4be1ff0..2cd9eebd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15530&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15530&range=00-01

  Stats: 111 lines in 4 files changed: 59 ins; 47 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/15530.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15530/head:pull/15530

PR: https://git.openjdk.org/jdk/pull/15530