Proposal: Low level struct improvements (#3936)

* Proposal: Low level struct improvements

This design covers adding the following features to C# for low level
struct performance improvements:

- Allowing `ref struct` to contain `ref` fields.
- Allowing `struct` and `ref struct` to return `ref` to their fields.
- Allowing safe fixed buffers for unmanaged and managed types.

Note: the section on providing parameter escape notations is incomplete
at this time. I will send a PR in the future once I finalize this
portion. Feel free to still leave comments though.

Iterations

* Rename to ThisRefEscapes

The name `[RefEscapes]` was causing a lot of confusion in the
discussions. People were mis-interpretting it as a general feature that
could apply to other constructs when in reality it's a very specific and
targetted feature. Renaming to `[ThisRefEscapes]` to make it clear this
is a very targetted feature.

* Allow heap inputs

Based on feedback from several people I've decided that we need to make
the rules for `ref` fields more flexible when the values being passed
around are known to refer to the heap. In those cases there is simply no
reason to restrict the created `ref struct` as it is always
*ref-safe-to-escape* to the enclosing method as well as not requiring
any adjustment to the method invocation rules.

This does mean that we need to add the notion of "refers to the heap" to
the span safety document. That doesn't change any assumptions there.
It's just that before this change the notion of "refers to the heap"
wasn't necessary to describe our safety rules.


Co-authored-by: Ruikuan <liruikuan@outlook.com>
Co-authored-by: Joseph Musser <me@jnm2.com>