implement cast expression
make model creation fail if head variable ill-formed
refactor boolean expression visitor to just expression
fix a string bug probably fixed elsewhere
make head expressions more expressive

specifically just allow a single binary op.
still not fully general; but this can be expanded when needed
for datalog, change project creation to apply

also make clang languages CApply compilation work properly.
Include test case for datalog parse of it
fix literal string warnings
add a clang test for binary ops in the IDB head
put project back in datalog model

for now Apply will also do column select, although
it may not have to
groupby in clang
test groupby
for group by nothing case, optimize with no map
tests for groupby clang
wip groupby grappa
address comment on fluffy code
Check for schema matching for binary bag/set opertions
Test binary bag/set operations
myrial_test: add a few more helper functions

Before, we could only execute queries. Now we can also get plans of
various types directly.
ensure SchemaMismatchException on logical plan
TDD tests for schema issues in while loops