[Issue 23321] New: Add ability to set NO_SCAN flag for GCAllocator

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23321

          Issue ID: 23321
           Summary: Add ability to set NO_SCAN flag for GCAllocator
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: schveiguy at gmail.com

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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