Optimize WeakReferenceMessenger

[Sergio0694]

Closes #40

This PR includes the following changes:

• WeakReferenceMessenger.Send is now zero-alloc 🚀
• WeakReferenceMessenger.Send is now also ~2.4x faster 🚀
• Better collision resistance for registered handlers
• Improved lookup speed when using custom tokens
• Remove UB due to Unsafe.As<T>(object) used to alias delegate types (see [API Proposal]: RuntimeHelpers.ChangeType<TDelegate>(Delegate) API dotnet/runtime#57495)

Benchmarks

Info	Mean	Error	StdDev	Gen 0	Allocated
main	17.41 ms	0.2930 ms	0.2600 ms	31.2500	188 KB
ConditionalWeakTable2 fork (dabca2a)	17.84 ms	0.0820 ms	0.0690 ms	31.2500	188 KB
ref struct enumerator (9b86a88)	15.32 ms	0.0730 ms	0.0570 ms	-	-
No active enumerators tracking (b1dc97b)	15.25 ms	0.0260 ms	0.0200 ms	-	-
Monitor.Enter loop hoisting (e92c5c2)	11.33 ms	0.2090 ms	0.1960 ms	-	-
Dictionary2 fork (9be01fa)	11.52 ms	0.0370 ms	0.0290 ms	-	-
typeof(Unit) specialization (fc538c3)	9.839 ms	0.1863 ms	0.1743 ms	-	-
Remove Unsafe.As(object) (9dd8ea9)	9.830 ms	0.1930 ms	0.1982 ms	-	-
No bounds checks, strength reduction (4408379)	9.309 ms	0.1709 ms	0.1515 ms	-	-
Remove KeyValuePair<,> usage	7.980 ms	0.1586 ms	0.2006 ms	-	-
Reduce enumerator field accesses (3f20dce)	7.348 ms	0.1494 ms	0.1834 ms	-	-
Remove array covariance checks (15bd652)	7.246 ms	0.1056 ms	0.0988 ms	-	-