FR: `expect_inherits()`

[t-kalinowski]

Now that base::inherits() can accept any object in the what argument, and S7 encourages using reified class objects instead of class names as character vectors, it might be useful to add an expect_inherits() function. This function would generally behave like expect_s3_class() but could dispatch nameOfClass() on the second argument when needed.

For example, it could replace tests like those seen here: cole-brokamp/fr#15

Possibly related issue: #1448

[hadley]

I feel like a more tactical change would be to implement expect_s7_class() because it has no implications for existing expect_s3_class() usage. It makes it more clear what the expectation does, and allows argument checking to be more precise.