Consider switching to using ValueTask across the library

[mikegoatly]

Mutation methods such as FullTextIndex.AddAsync are async only because it's possible that an index modification action may be present and may be async, e.g. serializing an index using async methods whenever an index is mutated.

This will save a few allocations per invocation to a mutation, though in the grand scheme of things may not be a big deal, so it needs to be measured.

[mikegoatly]

A quick experiment shows it makes next to no difference at all.

Before

Method	Mean	Error	StdDev	Rank	Gen0	Gen1	Gen2	Allocated
IndexingAlwaysSupportIntraNodeText	1.887 s	2.4127 s	0.1322 s	2	55000.0000	14000.0000	2000.0000	317.06 MB
IndexingIntraNodeTextAt4Characters	1.505 s	0.5183 s	0.0284 s	1	55000.0000	14000.0000	2000.0000	319.68 MB
IndexingOneByOneAlwaysSupportIntraNodeText	1.942 s	1.4130 s	0.0775 s	3	114000.0000	78000.0000	3000.0000	666.5 MB
IndexingOneByOneIntraNodeTextAt4Characters	2.056 s	1.3604 s	0.0746 s	4	115000.0000	77000.0000	3000.0000	674.7 MB

After

Method	Mean	Error	StdDev	Rank	Gen0	Gen1	Gen2	Allocated
IndexingAlwaysSupportIntraNodeText	1.879 s	1.0342 s	0.0567 s	2	55000.0000	14000.0000	2000.0000	317.06 MB
IndexingIntraNodeTextAt4Characters	1.484 s	0.3221 s	0.0177 s	1	55000.0000	14000.0000	2000.0000	319.68 MB
IndexingOneByOneAlwaysSupportIntraNodeText	1.948 s	0.3818 s	0.0209 s	4	114000.0000	78000.0000	3000.0000	666.5 MB
IndexingOneByOneIntraNodeTextAt4Characters	1.918 s	0.4472 s	0.0245 s	3	116000.0000	74000.0000	3000.0000	674.7 MB