[code-reflection] RFR: Descriptor to reference

[Paul Sandoz]

Refactor `{Method, Field, RecordType}Desc` to `{Method, Field, RecordType}Ref` and move them to the `types` package. Remove the `descriptor` package. Replace usages of the noun `descriptor` (except for now, in the context of reflective operations).

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Commit messages:
 - Update Triton example
 - Update tests
 - Refactor test
 - Refactory descriptors to references

Changes: https://git.openjdk.org/babylon/pull/33/files
 Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=33&range=00
  Stats: 1164 lines in 37 files changed: 435 ins; 475 del; 254 mod
  Patch: https://git.openjdk.org/babylon/pull/33.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/33/head:pull/33

PR: https://git.openjdk.org/babylon/pull/33