`FuncPtr` interop: support more than 16 arguments

In #354, we've introduced `FuncPtr` interop: C code may call .NET Core and pass function pointers in the positions when `FuncPtr` is expected.

Currently, we are limited to 16 parameters for each such function because there are no longer delegate types in .NET runtime.

We should support functions of any length instead, perhaps by inventing a custom calling convention on `ValueTuple`s.

While implementing it, seek for number `493` in the code.