Extend array initializers to stackalloc arrays

[BillWagner]

This feature enables array initializer syntax to be used with stackalloc arrays:

stackalloc int[3]				// currently allowed
stackalloc int[3] { 1, 2, 3 }
stackalloc int[] { 1, 2, 3 }
stackalloc[] { 1, 2, 3 }

The proposal for this feature is here

[BillWagner]

There are two articles that need to be updated for this feature:

• The language reference for stackalloc
• The conceptual topic for value types and reference semantics

Both have samples that should be updated with new examples that demonstrate the new feature.