Insert explicit byte-pointer casts for void pointer arithmetic

[tannergooding]

C/C++ allow pointer arithmetic on void* (treating it as byte-sized), but C# does not and instead reports CS0242: The operation in question is undefined on void pointers. The generator emitted the arithmetic verbatim, producing uncompilable C# for a function body such as cpy(buf + size * size2, info, size).

This inserts an explicit (byte*) cast on any void* operand of an additive operator so the arithmetic is well-defined; byte* implicitly converts back to void* where the surrounding context requires it.

cpy(buf + size * size2, ...)  ->  cpy((byte*)buf + size * size2, ...)

The cast is applied to whichever operand is the void* (LHS or RHS), so buf - n, n + buf, and pointer difference a - b are all handled.

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Compound assignment (+=/-=) and ++/-- on a void* lvalue are out of scope here -- those need a buf = (void*)((byte*)buf + n) rewrite rather than a localized operand cast, and are not exercised by the reported repro. Left as a follow-up.

Added a C regression test (CTest.VoidPointerArithmeticTest); C++ makes void* arithmetic a hard error, so the test uses C mode. No existing golden output changes -- the full ClangSharp.PInvokeGenerator.UnitTests suite passes (Failed: 0, Passed: 3725).

Fixes #296