RFR: API classes supporting RuntimeTypes and ArrayCreation bootstraps [v3]

[Dan Smith]

> - Introduce the RuntimeType interface
> - Modify Class to implement RuntimeType
> - Introduce NullRestrictedClass as another implementation of RuntimeType
> - Provide ConstantDesc support for NullRestrictedClasses
> - Introduce reflection hooks to query RuntimeType of fields and arrays (just return Classes for now)
> - Provide ArrayCreation bootstraps to create arrays with RuntimeTypes, initial values, and flags (no runtime support for new features yet—implementation bottoms out at Array.newInstance)
> - Basic functionality testing of ArrayCreation bootstraps

Dan Smith has updated the pull request incrementally with two additional commits since the last revision:

 - ArrayCreation.makeDefault: throw AssertionError, not IllegalArgumentException
 - Simplified primitive 'makeComputed' methods

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1210/files
  - new: https://git.openjdk.org/valhalla/pull/1210/files/2c86ac12..5a5e9b13

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1210&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1210&range=01-02

  Stats: 25 lines in 1 file changed: 16 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/valhalla/pull/1210.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1210/head:pull/1210

PR: https://git.openjdk.org/valhalla/pull/1210