Specialize tuple converter up to size 4

[Tarmil]

Implement specialized converters for struct and ref pairs, triplets and quadruplets. This avoids reflection, saving all extraneous allocations and some time too.

For example when deserializing an (int * bool) pair:

Method	Mean	Error	StdDev	Gen0	Allocated
StructGeneric	409.1 ns	1.59 ns	1.48 ns	0.0081	112 B
StructSpecialized	367.2 ns	1.57 ns	1.39 ns	-	-
RefGeneric	417.9 ns	1.25 ns	1.11 ns	0.0081	112 B
RefSpecialized	383.0 ns	1.47 ns	1.23 ns	0.0014	24 B