Import constraints from SQL files

[sekruse]

This issue is basically a modified version of #32, only that these constraint types should be loaded from a SQL file rather than a RDBMS. Particular care should be taken, that constraints can be defined with different SQL constructs, e.g., in CREATE TABLE, CONSTRAINT, ALTER TABLE ALTER COLUMN, and ALTER TABLE ADD CONSTRAINT (and maybe some more). Some research will be required to determine the relevant constructs there.

CHECK constraints with user-defines predicates are out of scope of this issue.