Optimize storage of default values in VerkleNode

[acolytec3]

Optimizes how we store default values for InternalNode and LeafNode so memory footprint is shrunk.

For InternalNode, if an element of internalNode.children has not been written to, the value is stored as null.

For LeafNode, if an element of the values array is 0 and has not been written before, it's stored as 0. If it's 0 and has been deleted, it's stored as 1. Otherwise, it's stored in its 32 byte Uint8Array representation.

Have also added some helpers to create default values (empty leafnode value, deleted leafnode value)

[holgerd77]

Nice, LGTM!

[acolytec3]

Need to fix some bugs locally before we merge. Just noticed I wasn't handling some of the changes correctly in the put method.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 19.51%. Comparing base (d24ca11) to head (bb7d30c).
Report is 44 commits behind head on master.

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HEAD has 2 uploads less than BASE

Flag	BASE (d24ca11)	HEAD (bb7d30c)
blockchain	1	0
statemanager	1	0

Additional details and impacted files

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block	72.71% <ø> (?)
blockchain	?
client	0.00% <ø> (?)
common	94.10% <ø> (-0.20%)	⬇️
devp2p	0.00% <ø> (?)
statemanager	?
tx	78.38% <ø> (?)
util	69.68% <ø> (-11.75%)	⬇️
wallet	0.00% <ø> (?)

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