Fix level when going down & in computeHashes, this affected the methods
Get, GenProof & AddBatch.

Update Snapshot & Root approach to get the root always from the db,
except in the cases that the tree is a snapshot, in which the root will
be in memory.
In this way, when a snapshot is performed and the original tree gets
modifyed, the snapshot will still point to the old root. Also, the root
obtained from the db, uses also the db.ReadTx, so if the root is being
modifyied in the current tx (db.WriteTx), when getting the root it will
be return the lastest version that is in the tx but not yet in the db.

Circom Verifier Proofs allow to verify through a zkSNARK proof the
inclusion/exclusion of a leaf in a tree. This commit adds the needed
code in go to generate the circuit inputs for a CircomVerifierProof.

Case tree empty, AddBatch was 10.95x times faster than without AddBatch
	nCPU: 4, nLeafs: 1024, hash: Poseidon, db: memory
	dbgStats(hash: 2.047k, dbGet: 1, dbPut: 2.049k)
Case tree not empty w/ few leafs, AddBatch was 7.28x times faster than without AddBatch
	nCPU: 4, nLeafs: 1024, hash: Poseidon, db: memory
	dbgStats(hash: 2.047k, dbGet: 198, dbPut: 2.049k)
Case tree not empty w/ enough leafs, AddBatch was 5.94x times faster than without AddBatch
	nCPU: 4, nLeafs: 1024, hash: Poseidon, db: memory
	dbgStats(hash: 2.047k, dbGet: 1.000k, dbPut: 2.049k)
Case tree not empty, AddBatch was 9.27x times faster than without AddBatch
	nCPU: 4, nLeafs: 4096, hash: Poseidon, db: memory
	dbgStats(hash: 8.191k, dbGet: 1.800k, dbPut: 8.193k)
Case tree not empty & unbalanced, AddBatch was 10.67x times faster than without AddBatch
	nCPU: 4, nLeafs: 4096, hash: Poseidon, db: memory
	dbgStats(hash: 10.409k, dbGet: 2.668k, dbPut: 10.861k)
TestAddBatchBench: nCPU: 4, nLeafs: 50000, hash: Blake2b, db: badgerdb
	Add loop:	10.10829114s
	AddBatch:	732.030263ms
		dbgStats(hash: 122.518k, dbGet: 1, dbPut: 122.520k)
TestDbgStats
	add in loop in emptyTree dbgStats(hash: 141.721k, dbGet: 134.596k, dbPut: 161.721k)
	addbatch caseEmptyTree dbgStats(hash: 24.402k, dbGet: 1, dbPut: 24.404k)
	addbatch caseNotEmptyTree dbgStats(hash: 26.868k, dbGet: 2.468k, dbPut: 26.872k)

- Update check of already existing key on add
- Update docs of methods

- Update public methods signatures
- Remove 'lastAccess' param
- Add option to pass root for tree.Dump, Iterate, IterateWithStop,
Graphviz (and related)
- Move error messages to const defined error messages for external usage

TestAddBatchBench
	nCPU: 4, nLeafs: 50000, hash: Blake2b, db: leveldb
		Add loop:	9.304195479s
		AddBatch:	817.590526ms

- Remove cropping on power of two the kvs for AddBatch
  - As not needed to be power of two length with the Virtual Tree
- Fix keyValuesToKvs keyPath

- AddBatch use Virtual Tree for cases A,B,C
- ImportDump use AddBatch instead of adding one by one
- Reorg & add more virtual tree tests

- Replace naive AddBatch by optimized AddBatch
- Add blake2b hash support
- Expose needed methods for external usage (ReadLeafValue,
ReadIntermediateChilds)
- Return 'value' in GenProof

CASE D: Already populated Tree
==============================
- Use A, B, C, D as subtree
- Sort the Keys in Buckets that share the initial part of the path
- For each subtree add there the new leafs

              R
             /  \
            /    \
           /      \
          *        *
         / |      / \
        /  |     /   \
       /   |    /     \
L:    A    B   C       D
     /\   /\  / \     / \
    ...  ... ... ... ... ...

buildTreeBottomUp splits the key-values into n Buckets (where n is the
number of CPUs), in parallel builds a subtree for each bucket, once all
the subtrees are built, uses the subtrees roots as keys for a new tree,
which as result will have the complete Tree build from bottom to up,
where until the log2(nCPU) level it has been computed in parallel.

As result of this, the tree construction can be parallelized until
almost the top level, almost dividing the time by the number of CPUs.