Eliminate allocs in TypeConverterCache lookup path

[MattKotsenas]

Switching the TypeConverterCache from a Dictionary to a ConcurrentDictionary accidentally re-introduced a Func<T> allocation on every .GetOrAdd() call.

Remove that allocation in the hot path by calling LookupTypeConverter statically. This saves ~5MB of allocations in the serialization benchmark.

Before

Method	Job	Runtime	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
Serializer	MediumRun-.NET 8.0	.NET 8.0	41.14 ms	1.280 ms	1.876 ms	1500.0000	500.0000	-	19.02 MB
Serializer	MediumRun-.NET Framework 4.7	.NET Framework 4.7	118.00 ms	6.116 ms	8.965 ms	6600.0000	600.0000	200.0000	41.34 MB

After

Method	Job	Runtime	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
Serializer	MediumRun-.NET 8.0	.NET 8.0	39.80 ms	1.457 ms	2.181 ms	1000.0000	500.0000	-	14.45 MB
Serializer	MediumRun-.NET Framework 4.7	.NET Framework 4.7	116.52 ms	5.888 ms	8.813 ms	5800.0000	600.0000	200.0000	36.75 MB

[EdwardCooke]

Awesome, thanks for the additional performance improvement work.