Adds FixedSizeBuilder type

The existing ImmutableArray<T> builder pattern through the Builder type
uses a minimum of two array allocations to create a single
ImmutableArray<T> instance.  The ImmutableArray<T>.Builder type has an
array field which is manipulated in much the same way as the array
inside of List<T>.  The call to ToImmutable on the builder creates a
copy of this array at the current length and wraps the result in an
ImmutableArray<T>

This pattern is great for a number of sceanrios.  For example when the
length of the array is variable or the application has low memory
pressure.

There are many situations though where a no copy / single allocation
solution would be advantageous.  In particular where there are a known
fixed number of elements to be converted to an array.  There is no need
for a double copy solution here because the final size is known in
advance.  All that is needed is for the array to be created, populated
and then frozen in an ImmutableArray<T> instance.

A good example of this need is Roslyn within Visual Studio.  Roslyn is
very careful to avoid unnecessary allocations, especially on hot paths.
Many of the hot paths include building instances of ImmutableArray<T>.
To avoid the double allocation Roslyn pools ImmutableArray<T>.Builder
instances.  This successfully avoids the double allocation but increases
the overall Gen2 pressure in the system.

This PR proposes to fix this by adding a no copy builder pattern:
FixedSizeBuilder.  This is a very simple wrapper around T[] which allows
for the creation and population of the array without the ability to
store a long term reference to the underlying array.  Hence it can be
safely converted to an ImmutableArray<T> without the need to copy the
array.

The builder can be reused for subsequent ImmutableArray<T> builds via
the Reset method
I have an idea... I suppose the primary scenario where perf for the index setter would really suffer from a simple `if` check is when the caller has very very little else to do but copy data. And *copying* data is a manual process right now for the caller.
So why don't we add a CopyTo method (and other methods commonly found on `Array`) to very rapidly copy into our private array from another array (and possibly other sources)?
That way, we could skip the `if` check inside the loop within our own method.
Or, since we know we *can* do that to solve a perf problem, we could just keep the if check here for now, and add `CopyTo` later if needed.