[code-reflection] RFR: Bytecode round 9

[Adam Sotona]

Bytecode round 9 contains following improvements:
- `String` (and non-`String`) concats are lifted to `CoreOp.ConcatOp`
- `LocalsTypeMapper` is fixed to use instruction index as a key (instead of the instruction identity)
- `LocalsTypeMapper` newly calculates types of uninitialized stack map frame entries and provides that info to `BytecodeLift`
- `BytecodeLift` is fixed to handle deep stacks correctly while branching + tests added
- Oboslete `BytecodeGenerator::conversion` method is replaced by direct call of `CodeBuilder::conversion`

`TestSmallCorpus` is not experiencing any first-lift exceptions after the above changes, however we cannot yet say that all the lifted models are correct.

Please review.

Thanks,
Adam

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Commit messages:
 - removed obsolete BytecodeGenerator::conversion
 - removed debug output
 - BytecodeLift fixes of code branching with deep stack and unitialized frame objects
 - fixed LocalsTypeMapper to track uninitialized types
 - fixed LocalsTypeMapper and lambdas lifting
 - fixed generating of non-String concats
 - lifting concats

Changes: https://git.openjdk.org/babylon/pull/192/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=192&range=00
  Stats: 231 lines in 5 files changed: 131 ins; 78 del; 22 mod
  Patch: https://git.openjdk.org/babylon/pull/192.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/192/head:pull/192

PR: https://git.openjdk.org/babylon/pull/192