Extract validateFeature and remove duplicate valueFromString
Fix PrimitiveDictionary arbitrary
Set tombstoneValue for dense row dictionaries later
Use Maps.outerJoin for top-level dictionary validation
Add virtual features to Dictionary

This has _no_ support for importing, but is just the start of internal support.
Merge pull request #218 from ambiata/topic/dictionary-virtual-feature

Add virtual features to Dictionary
Crash error module encapsulating sys.error
Replacing sys.error implementations with Crash.error
Merge pull request #221 from ambiata/topic/error-module

Error module for crashing ivory.
Rename Feature Meta/Virtual to Definition

- Also removed default type for tombstone
Merge pull request #222 from ambiata/topic/dictionary-definition-rename

Rename Feature Meta/Virtual to Definition
added fromResultTIO