MTP: Extension node is a lie!

[chfast]

MPT: Remove explicit extended node kind

Notice that the formally specified extended node in the Merkle Patricia Trie always leads to a branch node. Therefore, we can treat branch nodes as having potentially non-empty "extended path" and remove the explicit extended node kind. This significantly simplifies the implementation.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.29%. Comparing base (b1f68a3) to head (2314743).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #703   +/-   ##
=======================================
  Coverage   94.29%   94.29%           
=======================================
  Files         143      143           
  Lines       16145    16110   -35     
=======================================
- Hits        15224    15191   -33     
+ Misses        921      919    -2     

Flag	Coverage Δ
eof_execution_spec_tests	16.47% <100.00%> (-0.16%)	:arrow_down:
ethereum_tests	26.76% <100.00%> (-0.15%)	:arrow_down:
ethereum_tests_silkpre	18.52% <100.00%> (-0.16%)	:arrow_down:
execution_spec_tests	17.55% <100.00%> (-0.17%)	:arrow_down:
unittests	89.72% <100.00%> (-0.01%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
test/state/mpt.cpp	100.00% <100.00%> (+1.81%)	:arrow_up:

[chfast]

After extracting refactoring to #751, this is finally ready for review.