Refactors the package serialization format

This new version is actually closer to what GNU R does in terms
of lazy loading of package elements, but differs in the implementaiton.

Package symbols are serialized to an 'environment' file on the classpath,
and the values serialized either to disk or inline within the index,
depending on the size of the serialized value (threshold is currently
1024 bytes)
Tweaked the RDataReader for efficiency

- Avoid the creation of the Flags object for each item
  read
- Converted recursive reading of PairLists to a for loop