Compiler doesn't take readonly members into account for span safety rules

[tannergooding]

The language currently disallows you from passing a Span<T> to an instance method of a ref struct, unless that ref struct is also readonly or an in extension method.

The compiler should presumably also take into consideration whether a given member is itself readonly on a mutable instance struct and allow that as well.

For reference: https://sharplab.io/#v2:EYLgtghgzgLgpgJwDQxASwDYB8ACAGAAhwEYBuAWACgcBmAhOCAEwHsA7DAT3rgDMDYCAK4BjGAQDKAHgAqAPgIBvKgVVE6OACwEAsgAoJABwhtZCgB4BKFWuWU1BAL5VnlKrR79Bo8RIBMZko2qh4MzOxcRNr6RiaBVsFB9mquru4axABsRH4EAMJUdg4eQmxQELxwRFlRBAByBlLAnPAKwNbJqkUOarGmAEItcBYEALwCMBAiANYQGBgsIgTN8ADaAKwAuhSdPcAAdPpWOw6uDoklZRVVJNla9QYBK8PLHQ7dPZLGA0Mj47BTWbzRbLIYbbaJBwAeihBAAouZDHAxHAmAQYCwCAB3FgIaZIARoNgiKr6SwENBQAgmNgsSbwNEYgi08RgIT0qowAAWVSJAOJcCQkLUMOpbEZPIYvFxVREJgA5OJYDKCOZhaoDkdLCcUi4gA

[tannergooding]

CC. @RikkiGibson, @jaredpar

[tannergooding]

Rikki suggested I also tag @VSadov, offline.

[RikkiGibson]

When I first heard this I thought it was related to #34487 but it looks like spilling is not involved. So the fix shouldn't be too complicated.