Add phpstan/phpstan-strict-rules

[msvrtan]

Install and run them

[codecov-io]

Codecov Report

Merging #70 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master     #70   +/-   ##
========================================
  Coverage      41.3%   41.3%           
  Complexity      315     315           
========================================
  Files            63      63           
  Lines          1070    1070           
========================================
  Hits            442     442           
  Misses          628     628

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 548ddd5...bd2e65f. Read the comment docs.