install_test must rebuild before testing.
Modifications to speed up constructTree.
Cache the results of regexp processing.
Removed unused variable.
Simplified the cloning code for Walker objects.
Use arrays instead of sets to check references.
Implemented _hasAttrs.
Use an object instead of an array.
Removed the old _keepAttrs, _cleanAttrs stuff.

Instead of creating a pattern that contains only attributes, the logic
has been changed so that we can just use the regular patterns. This
yields a speed improvement when loading and creating a tree.
Lazily construct Data and Value.
Prepare for next version.
Drop old cruft.
Version bump.
Merge branch 'release/0.19.0'
Merge tag 'v0.19.0' into develop

0.19.0