[phobos] Array preallocate function

[Steve Schveighoffer]

Now that the append patch is pretty much a lock in for the next version of phobos/druntime, we need to address losing preallocate functionality.

Specifically, the following code does not preallocate as it previously did:

auto buffer = new int[10000];
buffer.length = 0;

buffer will reallocate on the first append because this can be considered a case of stomping, thereby wasting the first allocation completely.

Therefore, we need a way to implement this behavior.  I think the best way to do it is a druntime function (also TBD is where it goes!) which needs to call a function in the low-level rt/lifetime.d module.  Here is a strawman to discuss, anyone has a better idea, please present it.  I have no emotional attachment to anything here:

T[] preAlloc(T)(size_t nElements);

as for a module for it to belong in, currently druntime has the following public modules:
bitop.d
cpuid.d
exception.d
memory.d
runtime.d
thread.d
vararg.d

And of course, there's object.di

My first intuition is to use memory.d, but that contains only one public item -- the GC struct.  However, this function is currently not a GC function (it is purposely not because then it can be GC agnostic, but I'm open to discussion on this).  Therefore, it probably should be outside the GC struct.

The only other place that makes sense to me is object.di.  And of course, it could live in its own module, but that might be overkill.

-------------------------

A further feature is to "reuse" a buffer.  For example:

auto buffer = new int[10000];
loop
{
   buffer.length = 0;
   // use append as necessary
}

Should we implement this as a function or should we refer users to the Appender in std.array (does it support this feature)?  With preallocation, we can guarantee no stomping, but with reusing a buffer, we cannot because we rely on the user to ensure nothing else has references to data in the buffer.

My intuition is that we should not supply this function, and refer users to the Appender struct (and ensuring it can do this).

-Steve