Fix Conversion Warnings with Conservative Marking

[Aidan63]

After a comment in #1339 this is the first of probably a few PRs where I'm going through the IMMIX GC with all of clangs conversion warnings set to errors and fixing them up.
In this case I've gone through the conservative marking function and what it calls to make them compile even if you enable all of clangs warnings as errors. There are a few things going on here.

• Curly bracket initialisation is now used, this forbids narrowing / lossy conversion which will silently happen other wise.
• uintptr_t is used instead of size_t for pointer manipulation. As of C++11 this type is the standard sanctioned way for pointer editing.
• C++ style casts are used instead of C-style.

For reference below is the clang pragma I used to enable all the warnings as errors, I put these just inside the MarkConservative function with a pragma pop at the end of the function to limit them to just that function and what it calls.

#pragma clang diagnostic push
#pragma clang diagnostic error "-Wconversion"
#pragma clang diagnostic error "-Wsign-conversion"
#pragma clang diagnostic error "-Wimplicit"
#pragma clang diagnostic error "-Wimplicit-int-conversion"
#pragma clang diagnostic error "-Wall"
#pragma clang diagnostic ignored "-Wunused-variable"
#pragma clang diagnostic ignored "-Wunused-but-set-variable"