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
To be clear, I wasn't advocating removing the index getter.
All the Builders in *this* library do act like collections in their own right. Being consistent within the library is more important than being consistent with other libraries, IMO. Even the existing ImmutableArray+Builder class implements the collection interfaces. 
Since this Builder is very strictly implemented as an underlying array, I think putting Add/Remove methods *might* be overkill (but I wouldn't object to the design if it did have them), but I think it should behave like a collection in other ways.
If you still disagree, we can wait for more input from others.