add support for at_least and at_most

[xneomac]

Add functions expect_at_least and expect_at_most as described in issue #79.

[lipanski]

thanks, looks good!

I need to see why those tests are failing before merging, maybe I'll have to increase the minimum supported Rust version or bump that crate.

[xneomac]

I have added a small modification.
Using the previous version I found something odd when using expect and expect_at_least (or expect_at_most).

mock.expect(2).expect_at_least(5).

Old behavior : the mock is expecting 5, not more.
New behavior : the mock is expecting 5 or more.

mock.expect(5).expect_at_most(2).

Old behavior : the mock is expecting 2, not less.
New behavior : the mock is expecting 2 or less.

[lipanski]

Released with 0.23.0