refactor: simplify item left/right refs

[forehalo]

change item.{left/right} from Option<Node> to Somr<Item>

[github-actions]

Benchmark for Linux

Click to view benchmark

Test	Base	PR	%
codec/jwst decode var_int (32 bit)	333.2±1.14µs	333.5±1.29µs	+0.09%
codec/jwst decode var_uint (32 bit)	287.3±0.65µs	287.5±0.94µs	+0.07%
codec/jwst decode var_uint (64 bit)	310.2±2.08µs	309.5±1.52µs	-0.23%
codec/jwst encode var_int (32 bit)	409.9±1.33µs	409.9±1.43µs	-0.00%
codec/jwst encode var_uint (32 bit)	270.7±1.59µs	273.6±4.69µs	+1.07%
codec/jwst encode var_uint (64 bit)	271.4±1.53µs	282.0±3.43µs	+3.91%
codec/lib0 decode var_int (64 bit)	268.5±0.47µs	271.7±1.98µs	+1.19%
codec/lib0 decode var_uint (32 bit)	241.5±2.82µs	242.1±2.95µs	+0.25%
codec/lib0 decode var_uint (64 bit)	233.9±1.66µs	233.9±2.00µs	-0.00%
codec/lib0 encode var_int (64 bit)	390.2±0.95µs	390.0±0.63µs	-0.05%
codec/lib0 encode var_uint (32 bit)	287.2±4.38µs	287.5±3.57µs	+0.10%
codec/lib0 encode var_uint (64 bit)	347.2±3.65µs	346.6±3.02µs	-0.17%
ops/array/jwst/insert	374.0±0.69µs	262.7±0.48µs	-29.76%
ops/array/jwst/insert range	505.7±2.50µs	384.7±0.43µs	-23.93%
ops/array/jwst/remove	263.3±0.31µs	170.3±0.10µs	-35.32%
ops/array/yrs/insert	80.1±0.36µs	81.9±0.41µs	+2.25%
ops/array/yrs/insert range	103.4±0.74µs	105.7±1.02µs	+2.22%
ops/array/yrs/remove	93.0±0.08µs	91.5±0.23µs	-1.61%
ops/text/jwst/insert	142.4±1.25µs	136.9±0.24µs	-3.86%
ops/text/jwst/remove	3.3±0.01µs	3.3±0.00µs	-0.00%
ops/text/yrs/insert	36.0±0.04µs	36.6±0.03µs	+1.67%
ops/text/yrs/remove	2.4±0.04µs	2.3±0.01µs	-4.17%
update/parse with jwst/basic.bin	65.9±0.16µs	67.1±0.11µs	+1.82%
update/parse with jwst/database.bin	53.3±0.10µs	54.3±0.10µs	+1.88%
update/parse with jwst/large.bin	4.9±0.05ms	5.1±0.05ms	+4.08%
update/parse with jwst/with-subdoc.bin	6.2±0.01µs	6.4±0.03µs	+3.23%
update/parse with yrs/basic.bin	42.6±0.06µs	42.2±0.13µs	-0.94%
update/parse with yrs/database.bin	34.4±0.05µs	33.9±0.08µs	-1.45%
update/parse with yrs/large.bin	3.2±0.03ms	3.1±0.02ms	-3.12%
update/parse with yrs/with-subdoc.bin	5.4±0.01µs	6.1±0.01µs	+12.96%

Benchmark for Windows

Click to view benchmark

Test	Base	PR	%
codec/jwst decode var_int (32 bit)	441.8±9.95µs	433.2±10.28µs	-1.95%
codec/jwst decode var_uint (32 bit)	533.1±22.30µs	527.0±18.63µs	-1.14%
codec/jwst decode var_uint (64 bit)	585.2±13.54µs	581.6±15.00µs	-0.62%
codec/jwst encode var_int (32 bit)	561.7±11.16µs	553.1±10.22µs	-1.53%
codec/jwst encode var_uint (32 bit)	366.6±9.13µs	359.2±8.82µs	-2.02%
codec/jwst encode var_uint (64 bit)	365.7±7.70µs	360.9±7.73µs	-1.31%
codec/lib0 decode var_int (64 bit)	352.3±8.29µs	347.0±7.99µs	-1.50%
codec/lib0 decode var_uint (32 bit)	294.3±7.62µs	293.0±8.53µs	-0.44%
codec/lib0 decode var_uint (64 bit)	294.4±7.90µs	294.5±9.57µs	+0.03%
codec/lib0 encode var_int (64 bit)	724.3±16.11µs	717.4±18.69µs	-0.95%
codec/lib0 encode var_uint (32 bit)	485.2±10.88µs	479.1±8.64µs	-1.26%
codec/lib0 encode var_uint (64 bit)	468.4±9.56µs	471.2±17.66µs	+0.60%
ops/array/jwst/insert	543.2±50.11µs	356.9±8.57µs	-34.30%
ops/array/jwst/insert range	649.7±13.96µs	502.4±11.82µs	-22.67%
ops/array/jwst/remove	316.8±4.94µs	208.5±4.98µs	-34.19%
ops/array/yrs/insert	129.1±2.26µs	134.2±2.51µs	+3.95%
ops/array/yrs/insert range	176.7±2.87µs	178.6±3.61µs	+1.08%
ops/array/yrs/remove	189.6±4.75µs	191.4±4.46µs	+0.95%
ops/text/jwst/insert	224.5±4.08µs	211.2±5.40µs	-5.92%
ops/text/jwst/remove	4.5±0.10µs	5.4±0.07µs	+20.00%
ops/text/yrs/insert	55.8±1.06µs	57.5±1.23µs	+3.05%
ops/text/yrs/remove	4.4±0.11µs	5.6±0.13µs	+27.27%
update/parse with jwst/basic.bin	121.5±2.98µs	118.4±5.95µs	-2.55%
update/parse with jwst/database.bin	95.7±2.10µs	93.6±3.20µs	-2.19%
update/parse with jwst/large.bin	5.6±0.30ms	5.7±0.32ms	+1.79%
update/parse with jwst/with-subdoc.bin	19.0±0.36µs	17.9±0.29µs	-5.79%
update/parse with yrs/basic.bin	80.5±2.03µs	79.1±1.44µs	-1.74%
update/parse with yrs/database.bin	65.1±2.05µs	61.8±2.28µs	-5.07%
update/parse with yrs/large.bin	3.9±0.14ms	3.8±0.12ms	-2.56%
update/parse with yrs/with-subdoc.bin	13.0±0.29µs	13.7±0.30µs	+5.38%