This PR changes the IL generated for stackalloc in the following way:

• If an existing optimization applies (loading byte arrays directly from PE data; using initblk when all bytes are the same), there is no change.
• Otherwise, if it's converted to ReadOnlySpan<T> and all initializers are constant, the whole stackalloc is replaced by a call to CreateSpan.
• Otherwise (e.g. when converted to Span<T> or when only some initializers are constant), CreateSpan is used as a part of regular stackalloc codegen, together with cpblk.

The IL for array initializers is also changed:

• If an array initializer for a type larger than byte with all initializers constant is converted to ROS, the whole expression is replaced with CreateSpan.

There are still some questions I have and things that would need addressing to make this code production quality:

1. Is it okay to call GetPinnableReference (without any pinning) to get the backing data for the result of CreateSpan, for use in cpblk?
2. Alignment. As I understand it, the compiler currently does not align the metadata blocks containing initialization data in any way. If this is a requirement, that would need to change.
3. ENC. This optimization does not work with ENC and the code needs to know whether to apply this optimization at the lowering stage. But I don't know how to find out if ENC is used from inside of LocalRewriter. Is there some way to do that?