BUG: fix concat to work with more iterables (GH8645)
closes #8645

Enhances `pd.concat` to work with any iterable (except specifically undesired ones like pandas objects and strings). A new test is included covering tuples, lists, generator expressions, deques, and custom sequences, and all existing tests still pass. Finally, a "what's new" entry is included (not sure this last is correct - but [pull requests guidelines](https://github.com/pydata/pandas/blob/master/CONTRIBUTING.md) mentioned it)
