Split the yaml API, document and final fixes

Description

This formalizes the new node API.

All public node-api related parts are now part of a node module. _yaml keeps the yaml-related parts.

Changes proposed in this merge request:

• Split Node and yaml APIs
• Document

Benchmarks

Those benchmarks are run against the latest master as an indication of what will the change be when merged

First one was run 6 times, dropping the first of each, on base-files.bst

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commit	action	python_version	max_memory	time
bschubert/node-api-split - 55931ca9	build - 12	py37	190	94.44
	build - 4	py37	190	104.56
	build - 8	py37	190	96.29
	show	py37	191	4.75
	show - cached	py37	191	4.82
master - 199f080f	build - 12	py37	184	94.36
	build - 4	py37	184	104.39
	build - 8	py37	184	95.4
	show	py37	162	5.43
	show - cached	py37	185	5.28

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Second one was run 12 times, dropping the first two of each, on debian-stack.bst

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commit	action	python_version	max_memory	time
bschubert/node-api-split - 55931ca9	show	py37	1616	66.42
master - 199f080f	show	py37	1550	70.79

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