Refactor constraints (#358)

Author: Tigrov

CHANGELOG.md

@@ -39,6 +39,7 @@
 - New #348: Realize `Schema::loadResultColumn()` method (@Tigrov)
 - New #354: Add `FOR` clause to query (@vjik)
 - New #355: Use `DateTimeColumn` class for datetime column types (@Tigrov)
+- Enh #358: Refactor constraints (@Tigrov)
 - New #356, #357: Implement `DMLQueryBuilder::upsertReturning()` method (@Tigrov)
 - Enh #356, #357: Refactor `Command::insertWithReturningPks()` and `DMLQueryBuilder::upsert()` methods (@Tigrov)
 - Enh #360, #361: Implement `DMLQueryBuilder::insertReturningPks()` method (@Tigrov)