RFR: 8259778: Merge MutableSpace and ImmutableSpace

[Kim Barrett]

Please review this change which merges ImmutableSpace into MutableSpace,
eliminating the former.  There were no interesting uses of ImmutableSpace,
other than as the base class for MutableSpace.  The name ImmutableSpace is
kind of a misnomer given that usage.

Testing:
mach5 tier1-3

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Commit messages:
 - remove immutablespace

Changes: https://git.openjdk.java.net/jdk/pull/2271/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2271&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259778
  Stats: 325 lines in 8 files changed: 51 ins; 258 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2271.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2271/head:pull/2271

PR: https://git.openjdk.java.net/jdk/pull/2271