Bug 23321 – Add ability to set NO_SCAN flag for GCAllocator

std.experimental.allocator.GCAllocator always allocates blocks that are marked as containing pointers for GC scanning. However, for something like an i/o buffer, there is no reason to scan this for pointers, since it's raw data from a file. There isn't an API that allows this, so I'm wondering if the GCAllocator should accept a template flag that dictates the NO_SCAN requirement? I did this for my own purposes for iopipe: https://github.com/schveiguy/iopipe/blob/6a8c10d2858f92978d72c55eecc7ad55fcc207e2/source/iopipe/buffer.d#L18

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