Extend node.dsleep() to support instant sleep.

[jmattsson]

Fixes #1776.

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution. I have not, but @eyaleb has quite comprehensively done it in There is a use case for a fast deep sleep feature #1776
• The code changes are reflected in the documentation at docs/en/*.

This introduces the "instant" deep sleep feature which Espressif added but largely hid away. See #1776 for further details. Instant deep sleep effectively shaves off ~100ms on the going-to-sleep, which can be quite significant for battery-powered devices.

Due to the lack of documentation, I haven't made the instant deep sleep the default since I don't fully know what the difference between the two deep sleep functions are.

[jmattsson]

Huh! I never knew it was possible to link into a pdf!

And I have no idea how you managed to commit onto my branch? I assume it's some github custom deviltry? :)

Learned something new here - cheers!

[marcelstoer]

I never knew it was possible to link into a pdf!

That feature has been around for many years (10ish?) but I don't know how widely adopted it's become in browsers and PDF readers/plugins.

I have no idea how you managed to commit onto my branch?

Cool, isn't it! When you create a PR GitHub offers this "Allow edits from maintainers" option (next to the green create-PR button), it's checked by default.

[jmattsson]

Ah, that's what that box does. I figured it was referring to the PR meta-info. Live & learn, eh? :)