Add to_a(sync)gen wrapper able to turn a sync generator function into its async variant and introduce an async version of os.walk.

[stankudrow]

Closes #160 .
Also may close #167 .

Features:

• add the to_agen decorator responsible for wrapping sync iterators and turning them into asynchronous ones - analogously to the to_coro decorator that makes a coroutine from a given function
• add an async version of the os.walk iterator with tests
• a couple of extra unrelated minor changes

[stankudrow]

#160 (comment) - the os.listdir already wrapped, the os.walk added.

[stankudrow]

@Tinche , hello. Rebased and ready for review, all checks have been passed.

Update: I found the issue #167 on an "invented-here" solution, what are your suggestions?

[Tinche]

Apologies. I have a small baby nowadays.

So the original sync os.walk is a generator, right? This has some very useful properties which we should keep. Can we make the async version an async generator? Each step can do a step of the sync one.

[stankudrow]

Apologies. I have a small baby nowadays.

So the original sync os.walk is a generator, right? This has some very useful properties which we should keep. Can we make the async version an async generator? Each step can do a step of the sync one.

Hello and no need for apologies, au contraire, congratulations.

[stankudrow]

Waits for the #199 to be merged.

[Martin005]

Waits for the #199 to be merged.

That PR has been merged now, can we please merge this one as well? 🙂

[stankudrow]

Waits for the #199 to be merged.

That PR has been merged now, can we please merge this one as well? 🙂

Good to go after the #213 .

[stankudrow]

Memo: the "README" file must be updated.