Add short types: nullable_t and non_nullable_t
rename {non_}nullable_t into {non_}nullable_data to suggest that they only classify 'get'-able values
add support for UNION, INTERSECT, EXCEPT and their UNION ALL variant

syntax:
  << union (t | t in $foo$; ...) (... | ...) >>
fix forgotten case in last commit

fixes #6
Add the SQL function: md5
Merge pull request #7 from jpdeplaix/md5

Add the SQL function: md5
Add the type for string arrays
Add the array_agg aggregator
Provide a polymorphic type to array_agg
Fix the internal type of array_agg and add a generic TArray type
Add all the other types of array handled by PGOCaml
Add the SQL operator: IN
Add a default field to OpTuple used when the right parameter is empty
IN: Unify types
Add a test case for IN
Fix the error message for IN
IN: Fix the test-case
Fix the name returned for arrays
Define always the same order for results
fix a typing bug in union-like operators