Introduce Cython for better performances

Description

Introduce Cython in order to speed up hot paths in BuildStream.

This rewrites part of _yaml.py and _variables.py to be in cython, offering a good speedup compared to the pure python version.

More rational is given on the ML: https://mail.gnome.org/archives/buildstream-list/2019-May/msg00023.html

TODO before merging

• Ensure Source distributions (./setup.py sdist) do contain the cython-generated ".c" files and do not need Cython nor install it.
• Rewrite commit messages with more insight and information
• Modify Node file_number,col and line to be int and ensure it is typed accordingly
• Document added files and classes
• Tackle all FIXMEs
• Tackle all TODOs
• Ensure coverage for cython files

Changes proposed in this merge request:

• Introduce pyproject.toml and isolated package builds in tox
• Rewrite _variables.py in Cython
• Rewrite _yaml.py in Cython
• Use C when calling _yaml stuff in _variables.py

Benchmarks

These benchmarks have been run on a lenovo T470 laptop, with 2 physical/4 virtual cores machine, with 32Go of DDR4 RAM, and a NVME SSD. The machine was running Fedora 30, fully updated as of the 22th of May, and had no other significant program running.

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commit	action	python_version	max_memory	time
bschubert/cython - 238458d2	build - 4	py37	176	150.98
	build - 8	py37	176	164.62
	show	py37	176	5.22
	show - cached	py37	176	5.35
master - 25172ed2	build - 4	py37	185	156.38
	build - 8	py37	185	171.02
	show	py37	162	8.7
	show - cached	py37	185	8.18

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