fix(rln): if only a single insertion, dont use batch_insert

[rymnc]

the batch_insert api is useful for multiple insertions while maintaining atomicity, but for single insertions, ruins performance - single insertions are atomic by
nature so this pr circumvents it.

[github-actions]

Benchmark for 58a46c8

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Test	Base	PR	%
FullMerkleTree::delete	1396.0±51.80ns	1416.7±49.03ns	+1.48%
FullMerkleTree::get	4.5±0.18ns	4.4±0.19ns	-2.22%
FullMerkleTree::override_range	3.4±0.10µs	3.7±0.05µs	+8.82%
FullMerkleTree::set	1417.7±37.42ns	1396.1±49.20ns	-1.52%
OptimalMerkleTree::compute_root	1471.6±81.05ns	1512.0±53.12ns	+2.75%
OptimalMerkleTree::delete	1412.5±63.63ns	1381.8±85.37ns	-2.17%
OptimalMerkleTree::get	29.0±1.95ns	27.5±0.99ns	-5.17%
OptimalMerkleTree::override_range	7.4±0.41µs	7.5±0.47µs	+1.35%
OptimalMerkleTree::set	1579.1±96.59ns	1567.4±85.91ns	-0.74%

[github-actions]

Benchmark for 58a46c8

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Test	Base	PR	%
Pmtree::compute_root	1.7±0.00ns	1.7±0.00ns	0.00%
Pmtree::get	456.6±0.28ns	465.2±0.23ns	+1.88%
Pmtree::override_range	177.5±6.44µs	188.3±18.24µs	+6.08%
Pmtree::set	67.9±0.07µs	67.7±0.42µs	-0.29%
Pmtree:delete	68.2±0.06µs	69.9±0.22µs	+2.49%

[github-actions]

Benchmark for d527e22

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Test	Base	PR	%
FullMerkleTree::delete	1317.8±93.48ns	1366.4±109.55ns	+3.69%
FullMerkleTree::get	4.0±0.28ns	4.2±0.25ns	+5.00%
FullMerkleTree::override_range	3.3±0.20µs	3.3±0.16µs	0.00%
FullMerkleTree::set	1359.7±82.41ns	1366.2±120.18ns	+0.48%
OptimalMerkleTree::compute_root	1578.1±124.44ns	1521.7±99.38ns	-3.57%
OptimalMerkleTree::delete	1598.9±100.26ns	1453.3±116.16ns	-9.11%
OptimalMerkleTree::get	32.2±2.30ns	31.7±3.05ns	-1.55%
OptimalMerkleTree::override_range	7.8±0.71µs	8.1±0.63µs	+3.85%
OptimalMerkleTree::set	1521.6±93.59ns	1387.2±92.57ns	-8.83%

[github-actions]

Benchmark for d527e22

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Test	Base	PR	%
Pmtree::compute_root	2.0±0.02ns	2.4±0.02ns	+20.00%
Pmtree::get	562.2±2.58ns	555.7±6.02ns	-1.16%
Pmtree::override_range	207.9±16.55µs	212.0±24.37µs	+1.97%
Pmtree::set	80.9±0.47µs	82.3±1.04µs	+1.73%
Pmtree:delete	81.0±0.58µs	82.8±1.07µs	+2.22%