Merge pull request #158 in G/fastr from ~MICK.JORDAN_ORACLE.COM/fastr:feature/rprof to master

* commit 'f9cd9395':
  Correct use of Source.internal()
  check style  fix; RPerfStats option fix
  Instrumentation refactoring. Retire RNodeTimer, REntryCounts; replace with RFunctionProfiler Retire FunctionUID replace use with SourceSection Refactor all context-specific instrumentation state into InstrumentationState class
  Enable rowsum test
  remove old FastROption
  row sum_matrix:  use AbstractVectors
  Rprof: move state into RContext
  Add support for storing Profiler in Context; more RSource cleanup/functions
  RowsumFunctions: cast pipeline review fixes
  Move RInternalInternalSourceDescriptions into RSource
  compiler fixes
  Tighten up Rsource methods, use getURI instead of getPath, set .internal() on internal sources Implement line.profiling
  Implement Rprof (no lineProfiling, memProfiling, gcProfiling)
  remove old  comment
  Implement rowsums_matrix (needed by summaryRprof); enable tests
  Change "description" to "name" in FunctionDefinitionNode. Fixes regression on source'd functions where name was lost (description is already used in Node for some other meaning)
  Provide access to Polyglot engine instance from Context