move exposed-provenance APIs into separate feature gate and explain the relationship of Exposed Provenance and Strict Provenance

Author: RalfJung

tests/fail/provenance/ptr_int_unexposed.rs

@@ -1,5 +1,5 @@
 //@compile-flags: -Zmiri-permissive-provenance
-#![feature(strict_provenance)]
+#![feature(strict_provenance, exposed_provenance)]
 
 fn main() {
     let x: i32 = 3;

tests/fail/provenance/ptr_invalid.rs

@@ -1,4 +1,4 @@
-#![feature(strict_provenance)]
+#![feature(strict_provenance, exposed_provenance)]
 
 // Ensure that a `ptr::invalid` ptr is truly invalid.
 fn main() {

tests/fail/provenance/strict_provenance_cast.rs

@@ -1,5 +1,5 @@
 //@compile-flags: -Zmiri-strict-provenance
-#![feature(strict_provenance)]
+#![feature(exposed_provenance)]
 
 fn main() {
     let addr = &0 as *const i32 as usize;

tests/fail/stacked_borrows/exposed_only_ro.rs

@@ -1,5 +1,5 @@
 //@compile-flags: -Zmiri-permissive-provenance
-#![feature(strict_provenance)]
+#![feature(exposed_provenance)]
 
 // If we have only exposed read-only pointers, doing a write through a wildcard ptr should fail.
 

tests/pass/ptr_int_from_exposed.rs

@@ -1,7 +1,7 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
 //@compile-flags: -Zmiri-permissive-provenance
-#![feature(strict_provenance)]
+#![feature(strict_provenance, exposed_provenance)]
 
 use std::ptr;
 

tests/pass/stacked-borrows/int-to-ptr.rs

@@ -1,5 +1,5 @@
 //@compile-flags: -Zmiri-permissive-provenance
-#![feature(strict_provenance)]
+#![feature(exposed_provenance)]
 use std::ptr;
 
 // Just to make sure that casting a ref to raw, to int and back to raw

tests/pass/stacked-borrows/unknown-bottom-gc.rs

@@ -1,5 +1,5 @@
 //@compile-flags: -Zmiri-permissive-provenance
-#![feature(strict_provenance)]
+#![feature(exposed_provenance)]
 
 use std::ptr;