[code-reflection] RFR: Assert code modeling

[Ian Graves]

Initial work on the code model for supporting assertions. This PR contains the code model. A subsequent PR will include interpreter updates to execute assertions in IR form.

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Commit messages:
 - Documentation for AssertOp
 - Assert code modeling

Changes: https://git.openjdk.org/babylon/pull/32/files
 Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=32&range=00
  Stats: 153 lines in 3 files changed: 153 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/babylon/pull/32.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/32/head:pull/32

PR: https://git.openjdk.org/babylon/pull/32