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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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%)	⬇️
ethereum_tests	26.76% <100.00%> (-0.15%)	⬇️
ethereum_tests_silkpre	18.52% <100.00%> (-0.16%)	⬇️
execution_spec_tests	17.55% <100.00%> (-0.17%)	⬇️
unittests	89.72% <100.00%> (-0.01%)	⬇️

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%)	⬆️

[chfast]

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

[gumb0]

What if we have a method to copy node with children, and try to simplify like:

Suggested change

	const Path extended_path{m_path.begin(), this_idx_it};
	const auto this_idx = *this_idx_it;
	m_path = Path{this_idx_it + 1, m_path.end()}; // shorten this path, invalidates this_idx_it
	*this = branch(extended_path, this_idx, std::make_unique<MPTNode>(std::move(*this)),
	*insert_idx_it, leaf(insert_tail, std::move(value)));
	// The original node must be pushed down.
	const Path common_path{m_path.begin(), this_idx_it};
	const Path this_node_tail{this_idx_it + 1, m_path.end()};
	auto this_node = copy_node(m_kind, this_node_tail, std::move(m_children), std::move(m_value)); // this_idx_it is not invalidated
	auto new_leaf = leaf(insert_tail, std::move(value));
	*this = branch(common_path, *this_idx_it, std::move(this_node), *insert_idx_it, std::move(new_leaf));

[chfast]

I don't think we need special method. The move constructor can do this. I believe I tried this but the effect was more code similarly confusing. I'm still thinking about some refactoring here (e.g. removing named constructors leaf() and branch()) but wanted to minimize the diff this time.