Extract `MutableJvmSymbol`

Naming the byte array as an explicit type gives us:
- one Javadoc to explain the contents
- memoized `new String(bytes)`

Also clean up dead code and add missing changelog.

JPERF-1454 Move rainbow to API, with a better name

TDD: refactor

JPERF-1409 Add MultiJfrFilter

Expose `RawCohortResult.cohort`