Fix json serialization and update corresponding test

[seemenkina]

Issue #252 and part of #237

[github-actions]

Benchmark for 57634a5

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Test	Base	PR	%
FullMerkleTree::compute_root	0.0±0.00ns	0.0±0.00ns	NaN%
FullMerkleTree::delete	852.4±22.61ns	844.9±9.51ns	-0.88%
FullMerkleTree::get	0.0±0.00ns	0.0±0.00ns	NaN%
FullMerkleTree::get_empty_leaves_indices	7.1±0.11ns	7.1±0.07ns	0.00%
FullMerkleTree::get_subtree_root	0.6±0.01ns	0.6±0.02ns	0.00%
FullMerkleTree::override_range	1310.8±41.60ns	1308.1±19.24ns	-0.21%
FullMerkleTree::set	857.1±9.43ns	843.0±7.77ns	-1.65%
OptimalMerkleTree::compute_root	998.5±22.79ns	1001.9±6.18ns	+0.34%
OptimalMerkleTree::delete	1006.8±17.11ns	1001.3±12.85ns	-0.55%
OptimalMerkleTree::get	23.6±0.18ns	23.8±0.30ns	+0.85%
OptimalMerkleTree::get_empty_leaves_indices	7.4±0.13ns	7.5±0.61ns	+1.35%
OptimalMerkleTree::get_subtree_root	24.3±0.39ns	24.6±0.45ns	+1.23%
OptimalMerkleTree::override_range	4.2±0.04µs	4.1±0.10µs	-2.38%
OptimalMerkleTree::set	1002.7±7.83ns	1004.1±33.05ns	+0.14%

[rymnc]

LGTM, thanks!

[github-actions]

Benchmark for 57634a5

Click to view benchmark

Test	Base	PR	%
FullMerkleTree::::full_height_gen	13.6±0.13ms	14.4±0.10ms	+5.88%
OptimalMerkleTree::::full_height_gen	564.5±10.90µs	574.4±5.04µs	+1.75%
Pmtree::compute_root	0.0±0.00ns	0.0±0.00ns	NaN%
Pmtree::get	338.7±4.70ns	331.0±3.67ns	-2.27%
Pmtree::get_empty_leaves_indices	8.4±0.14ns	8.4±0.09ns	0.00%
Pmtree::get_subtree_root	466.1±5.30ns	470.3±5.27ns	+0.90%
Pmtree::override_range	236.5±6.94µs	239.5±4.01µs	+1.27%
Pmtree::set	53.7±0.40µs	54.5±0.34µs	+1.49%
Pmtree:delete	53.7±0.32µs	54.6±0.57µs	+1.68%
Set/FullMerkleTree::::set/1	543.7±7.42µs	554.6±6.48µs	+2.00%
Set/FullMerkleTree::::set/10	5.5±0.02ms	5.5±0.02ms	0.00%
Set/FullMerkleTree::::set/100	54.7±0.06ms	55.6±0.20ms	+1.65%
Set/FullMerkleTree::::set_range/1	542.2±7.54µs	551.6±3.25µs	+1.73%
Set/FullMerkleTree::::set_range/10	748.6±6.84µs	757.9±5.64µs	+1.24%
Set/FullMerkleTree::::set_range/100	3.3±0.01ms	3.3±0.02ms	0.00%
Set/OptimalMerkleTree::::set/1	546.2±3.23µs	553.5±4.62µs	+1.34%
Set/OptimalMerkleTree::::set/10	5.5±0.02ms	5.5±0.02ms	0.00%
Set/OptimalMerkleTree::::set/100	55.2±0.11ms	55.6±0.17ms	+0.72%
Set/OptimalMerkleTree::::set_range/1	545.6±5.63µs	553.6±4.55µs	+1.47%
Set/OptimalMerkleTree::::set_range/10	5.5±0.02ms	5.6±0.04ms	+1.82%
Set/OptimalMerkleTree::::set_range/100	55.2±0.24ms	55.5±0.23ms	+0.54%
zkey::upload_from_folder	3.3±0.00s	3.3±0.00s	0.00%