Added === as explicit “IS” operator for expressions

[jake-b]

Sometimes, you explicitly want the "IS" operator, unlike the automatic logic that sometimes uses the IS operator when using the == operator with expressions.

Possible solution for feature request stephencelis/SQLite.swift#1003

• Added === as explicit “IS” operator for expressions.
• Added tests for === operator.

[nathanfallet]

@jake-b Thanks for your contribution. You can (if you want and if you have time to) add it to the documentation (to show it exists)

[jake-b]

Thank you. Please see #1060 for the quick update to the documentation.