expr: table and tablesymbol hold on to dshape, not schema

This allows for table length to be maintained
table: len
table: define len for various expressions
table: len(Table(...)) calls compute if necessary
expr: bool(TableExpr(...)) == True
expr: better error messaging behind length
Merge pull request #473 from mrocklin/table-length

Overload `__len__` to work on Table Expressions and on Table interactive objects