feat: Add benchmarking and significantly refactor based on results

[seanjnkns]

Add benchmarking tests and README documentation. Numerous significant improvements to code based on iterative runs of the benchmark, refactoring code and adding additional benchmarks, or tests.

Impact:

Benchmark Comparison Results

Benchmark	Test	Original ns/op	New ns/op	Δ ns/op	Original B/op	New B/op	Δ B/op	Original allocs/op	New allocs/op	Δ allocs/op
BenchmarkMustGetCurrentMappingState	small-8	11,434,218	9,528,867	-16.7%	3,138,058	1,867,982	-40.5%	52,195	37,119	-28.8%
BenchmarkMustGetCurrentMappingState	medium-8	30,417,606	26,208,713	-13.8%	12,461,741	7,479,941	-39.9%	208,433	148,203	-28.9%
BenchmarkMustGetCurrentMappingState	large-8	120,747,148	90,087,658	-25.4%	50,486,614	30,683,584	-39.2%	833,357	592,516	-28.9%
BenchmarkMustGetCurrentMappingStateLegacyJSON	small-8		9,690,610			1,867,926			37,117
BenchmarkMustGetCurrentMappingStateLegacyJSON	medium-8		26,129,703			7,479,904			148,201
BenchmarkMustGetCurrentMappingStateLegacyJSON	large-8		90,300,395			30,683,529			592,514
BenchmarkCalcPgMappingsToUndoBackfill	small-8	16,413,102	13,948,699	-15.0%	3,972,010	2,073,725	-47.8%	65,910	39,882	-39.5%
BenchmarkCalcPgMappingsToUndoBackfill	medium-8	47,989,834	34,761,084	-27.5%	15,257,223	8,302,134	-45.6%	263,117	159,162	-39.5%
BenchmarkCalcPgMappingsToUndoBackfill	large-8	172,888,991	113,219,697	-34.5%	61,118,536	34,078,135	-44.2%	1,051,888	636,248	-39.5%
BenchmarkCalcPgMappingsToDrainOsd	small-8	71,042,975	43,137,217	-39.3%	16,658,772	13,914,102	-16.5%	417,613	39,001	-90.7%
BenchmarkCalcPgMappingsToDrainOsd	medium-8	79,123,788	56,851,963	-28.1%	26,106,239	19,740,991	-24.4%	575,945	152,210	-73.6%
BenchmarkCalcPgMappingsToDrainOsd	large-8	191,583,932	153,921,553	-19.7%	65,207,584	43,825,932	-32.8%	1,209,341	605,007	-49.9%
BenchmarkCalcPgMappingsToUndoUpmaps	small-8	200,001,405	10,639,069	-94.7%	117,088,990	1,990,615	-98.3%	111,408	38,559	-65.4%
BenchmarkCalcPgMappingsToUndoUpmaps	medium-8	2,350,508,930	26,820,746	-98.9%	1,813,717,012	7,958,489	-99.6%	560,061	153,851	-72.5%
BenchmarkCalcPgMappingsToUndoUpmaps	large-8	24,504,742,986	95,959,828	-99.6%	28,742,183,301	32,625,441	-99.9%	3,945,517	615,004	-84.4%
BenchmarkCalcPgMappingsToBalanceOsds	small-8	13,269,599	9,268,311	-30.1%	3,170,515	1,902,283	-40.0%	52,414	37,338	-28.7%
BenchmarkCalcPgMappingsToBalanceOsds	medium-8	31,954,823	26,079,697	-18.4%	12,502,672	7,514,257	-39.9%	208,653	148,422	-28.8%
BenchmarkCalcPgMappingsToBalanceOsds	large-8	116,185,334	91,074,987	-21.6%	50,521,320	30,717,746	-39.2%	833,576	592,735	-28.9%
BenchmarkImportMappingsLoop	small-8	203,225,079	10,602,477	-94.8%	117,297,093	1,992,297	-98.3%	112,989	38,639	-65.8%
BenchmarkImportMappingsLoop	medium-8	2,314,901,014	27,110,779	-98.8%	1,813,358,903	7,967,329	-99.6%	566,646	154,231	-72.8%
BenchmarkImportMappingsLoop	large-8	24,220,767,435	98,135,411	-99.6%	28,741,009,808	32,663,195	-99.9%	3,972,295	616,584	-84.5%
BenchmarkExportMappings	small-8	203,832,152	10,275,006	-95.0%	117,198,357	1,991,624	-98.3%	111,314	38,572	-65.3%
BenchmarkExportMappings	medium-8	2,279,265,827	27,300,936	-98.8%	1,813,272,375	7,959,478	-99.6%	559,895	153,864	-72.5%
BenchmarkExportMappings	large-8	24,611,868,229	95,992,547	-99.6%	28,742,835,394	32,626,524	-99.9%	3,945,497	615,017	-84.4%
BenchmarkRemapSingle	small-8	13,245,572	10,297,120	-22.2%	3,151,222	1,868,113	-40.7%	52,208	37,122	-28.9%
BenchmarkRemapSingle	medium-8	31,505,698	26,007,101	-17.5%	12,474,771	7,480,049	-40.0%	208,446	148,206	-28.9%
BenchmarkRemapSingle	large-8	115,908,597	90,438,220	-21.9%	50,508,131	30,683,649	-39.2%	833,371	592,519	-28.9%
BenchmarkGenerateCrushChangeMappings	small-8	2,580,895	2,927,772	+13.4%	2,073,704	2,073,647	-0.0%	16,438	16,436	-0.0%
BenchmarkGenerateCrushChangeMappings	medium-8	10,202,671	10,862,061	+6.5%	8,302,299	8,302,252	-0.0%	65,688	65,686	-0.0%
BenchmarkGenerateCrushChangeMappings	large-8	47,727,619	47,607,074	-0.3%	33,206,249	33,206,194	-0.0%	262,682	262,680	-0.0%