RFR: 8269957: facilitate alternate impls of NameTable and Name

[Archie L. Cobbs]

The `Name.Table` class is used by the compiler to hold unique instances of strings as `Name` objects.

In theory the `Name` superclass supports alternate implementations beyond the two existing implementations (`SharedNameTable` and `UnsharedNameTable`), but its current design presumes that strings are stored as UTF-8 byte arrays, which discourages other approaches.

The goal of this PR is to refactor things to allow for more flexibility in alternate `Name` implementations.

As a simple test case of this idea, it should be relatively simple to implement a `Name.Table` that stores `String`s in a hash table. This patch includes such an example in the new class `StringNameTable`, which can be enabled via the `-XDuseStringTable=true` command line flag. 

I have not done any performance testing or comparisons of `StringNameTable`; see also [JDK-8268622](https://bugs.openjdk.org/browse/JDK-8268622) which argues that a `String`-based implementation should be faster.

Changes:
* Remove all byte-oriented methods from the `Name` and `Name.Table` API's, except for those that import/export Modified UTF-8.
* Change the semantics of `Name.subName()` so the offset is a character offset, not a byte offset.
* Consolidate the common UTF-8 machinery of `SharedNameTable` and `UnsharedNameTable` into a new common superclass `Utf8NameTable`.
* Rename `Name.lastIndexOf()` -> `Name.lastIndexOfAscii()` to more accurately reflect its expected behavior.
* Add new `StringNameTable` implementation.

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

Commit messages:
 - Merge branch 'master' into JDK-8269957
 - Move equals() table check into common superclass.
 - Add Name.Table factory methods to Names class.
 - Add StringNameTable as a new (optional) Name.Table implementation.
 - Add a few more default method implementations to Name superclass.
 - Revert some unnecessary changes.
 - Merge branch 'master' into JDK-8269957
 - Refactor Name and friends to facilitate future improvements.

Changes: https://git.openjdk.org/jdk/pull/13282/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13282&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8269957
  Stats: 835 lines in 11 files changed: 571 ins; 163 del; 101 mod
  Patch: https://git.openjdk.org/jdk/pull/13282.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13282/head:pull/13282

PR: https://git.openjdk.org/jdk/pull/13282