Improve operator==

operator== and operator!= can now compare besides two expressions, also an expression with a compatible container.

empty() function added.

Now expr.empty() works like empty() member function of an ordinary container.

v.1.1.0: Merge pull request #4 from tirimatangi/identity-expression

Add function asExpression for turning containers into expressions more easily.

Fix arithmetic expressions

This fix solves the Dec 1, 2021 issue. See Issues page.

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