Add {ReadOnly}Span ctor (internal for now) that takes a ref and no length

[stephentoub]

Contributes to #67445

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[jkotas]

_pointer needs to be also renamed in src\mono\mono\mini\intrinsics.c(330):MonoClassField *ptr_field = mono_class_get_field_from_name_full (cmethod->klass, "_pointer", NULL);