Redundant "Clear" in many buffer allocations

[mgravell]

In a number of places in the storage layer, GC.AllocateArray is used to allocate an array on the pinned heap, followed shortly by an Array.Clear. This is redundant, since GC.AllocateArray pre-initializes the data to zero. I'm guessing this might be a hangover from a previous unmanaged allocator, or from migration from a C/C++ implementation.

If the intent is to use unitialized data in the oversized portion: GC.AllocateUnitializedArray exists (but only works for blittable types); otherwise and probably simpler to grok (and easier to validate correctness): just remove the Array.Clear calls, leaving the default/inbuilt zero initialize (i.e. do not switch to GC.AllocateUninitializedArray).

impact: performance

examples:

• BlittableFrame.Allocate
• MallocFixedPageSize.InternalAllocate (2 of)
• SpanByteAllocator.AllocatePage

similar with simple array creation (no GC alloc):

• GenericAllocator<,>.AllocatePage
• GenericFrame<,>.Allocate