[code-reflection] RFR: Adding String Concatenation to Code Model

[Ian Graves]

Adding String concatenation to the code model and interpreter.

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Commit messages:
 - Test cleanups
 - Test tweak
 - Shortening up tests and minor tweaks
 - Initial commit

Changes: https://git.openjdk.org/babylon/pull/47/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=47&range=00
  Stats: 301 lines in 4 files changed: 299 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/babylon/pull/47.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/47/head:pull/47

PR: https://git.openjdk.org/babylon/pull/47