Versioned packages.

Each package now has a SxVersion constant. It is set to the Hex SHA1 of
all the sources files in the package. SxVersion allows sx to know when a
package changed and, as a result, which aggregates need to be
recalculated.

TODO:
- The transformations need to use SxVersion.
First import
First import
Removed lang (preparing for sx-lang-master merge)
Subtree merged in lang/