allow view to return generator

[pgjones]

Directly accept iterators/generators as returned from view functions

My expectation was that this was already possible to stream responses
and I can't think of a reason why this shouldn't be allowed. Quart
already does this without any issues I know of.

Add stream_template and stream_template_string functions

These helper functions make it easier to render a template piece by
piece with the correct context. Much like render_template and
render_template_string make it easy to render the entire template.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[davidism]

• Instead of using yield from directly, use a closure so that we can wrap it with stream_with_context automatically if a request is active. This still returns a generator, but the function itself is not a generator.
• Use isinstance(value, collections.abc.Iterator) instead of inspect.isgenerator(value). This allows returning iter(["a", "b", "c"]), as well as generators and generator expressions.
• Update the types and type tests to allow Iterator, which also allows Generator.