Integrate Psalm

[README1ST]

Signed-off-by: Adam Williams adam.williams3@sky.uk

Q	A
Documentation	~yes~/no
Bugfix	~yes~/no
BC Break	~yes~/no
New Feature	~yes~/no
RFC	~yes/no~/??
QA	yes/~no~

Description

Integrates Psalm.

Have followed a similar approach to a PR here: https://github.com/laminas/laminas-hydrator/pull/33

Variations from the recommendation in #8:

• Configuration file is psalm.xml.dist, not .psalm.xml.dist.
• Adds vimeo/psalm to composer require-dev and not require.
• Does NOT use phpunit plugin in configuration (caused Psalm to raise an exception, even when I had it installed).
• Does NOT scan bin, src or "test" directory (this repo does not have either). Instead I have set this to library and "tests".
• Does NOT remove phpstan integration as that did not exist to begin with.
• Does NOT update the CI as Travis integration did not exist to begin with.

Fixes #8

[boesing]

Thanks, @README1ST