mostly working toIterator
refactor to use TypedSequenceFile (single field), which requires having Manifests available

note: toIterator doesn't work for tuples, supposedly can't find deserializer, even though "write"/"save" both can deserialize, run things, and write back out
fix tuple reading for Hadoop mode
Merge branch 'repl+execution' into repl+toiterator
Merge branch 'repl+execution' into repl+toiterator
pattern match for head pipe with Converter
add TextLine.toIterator test
get rid of unnecessary implicit Manifest, TupleConverter, TupleSetters
add tests for toIterator, 'tuple' one currently breaks it because of serialization problem
snapshot: use MemorySink for Local mode, handle this case in toIterator
add implicit to enrich ValuePipe -> ShellPipe
test that implicit snapshot on `toIterator` works in all the cases
add comment explaining TypedSequenceFile
fixes and cleanup from @johnynek's feedback
document toIterator, toList, and dump
TypedPipe.from(List).distinct fails
Another test case
working on making running tests in Local & Hadoop modes
make ReplImplicit.{flowDef,mode} non-implicit, plumb implicit FlowDef & Mode through
different paths for local/hdfs modes