Add NullHeap

[roxblnfk]

NullHeap is a heap implementation that does nothing.
It is useful when you are using ORM only for reading and you don't need to cache the loaded data.

Effects of using NullHeap:

• Less memory consumption
• No need to clean the heap after each request in long-living applications
• You won't be able to update entities in the database. ORM will consider each loaded entity as newly created.

Fix #419

[codecov]

Codecov Report

Merging #441 (5bed70c) into 2.x (35bfbf1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                2.x     #441      +/-   ##
============================================
+ Coverage     91.60%   91.62%   +0.01%     
- Complexity     1766     1772       +6     
============================================
  Files           112      113       +1     
  Lines          4505     4514       +9     
============================================
+ Hits           4127     4136       +9     
  Misses          378      378              

Files	Coverage Δ
src/Heap/NullHeap.php	100.00% <100.00%> (ø)