Simplify Template API

[nijel]

Remove some features we do not seem to use:

• helper functions (this never worked with Twig)
• per class context, it now has to be passed to render
• this avoids need of array_merge call on every render

Signed-off-by: Michal Čihař michal@cihar.com

Before submitting pull request, please check that every commit:

• Has proper Signed-Off-By
• Has commit message which describes it
• Is needed on it's own, if you have just minor fixes to previous commits, you can squash them
• Any new functionality is covered by tests

[MauricioFauth]

The helper functions are not being used in phtml templates if I'm not mistaken and in Twig we use the extensions, so it's safe to remove this functionality.
And passing the data directly to the render makes more sense.

[codecov]

Codecov Report

Merging #13717 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #13717      +/-   ##
==========================================
- Coverage   53.75%   53.73%   -0.03%     
==========================================
  Files         481      481              
  Lines       82190    82150      -40     
==========================================
- Hits        44179    44141      -38     
+ Misses      38011    38009       -2