Safe Memory read/write API (#2528)

This commit introduces two new methods on `Memory` that enable
reading and writing memory contents without requiring `unsafe`.

The methods return a new `MemoryError` if the memory access
fails.

tests/all/externals.rs

@@ -340,3 +340,40 @@ fn grow_externref_tables_via_api() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn read_write_memory_via_api() {
+    let cfg = Config::new();
+    let store = Store::new(&Engine::new(&cfg));
+    let ty = MemoryType::new(Limits::new(1, None));
+    let mem = Memory::new(&store, ty);
+    mem.grow(1).unwrap();
+
+    let value = b"hello wasm";
+    mem.write(mem.data_size() - value.len(), value).unwrap();
+
+    let mut buffer = [0u8; 10];
+    mem.read(mem.data_size() - buffer.len(), &mut buffer)
+        .unwrap();
+    assert_eq!(value, &buffer);
+
+    // Error conditions.
+
+    // Out of bounds write.
+
+    let res = mem.write(mem.data_size() - value.len() + 1, value);
+    assert!(res.is_err());
+
+    // Out of bounds read.
+
+    let res = mem.read(mem.data_size() - buffer.len() + 1, &mut buffer);
+    assert!(res.is_err());
+
+    // Read offset overflow.
+    let res = mem.read(usize::MAX, &mut buffer);
+    assert!(res.is_err());
+
+    // Write offset overflow.
+    let res = mem.write(usize::MAX, &mut buffer);
+    assert!(res.is_err());
+}