rename the pointer read/write methods to read and write

these names were artifacts of some early confusion / bad design i made
in the traits. read and write are much simpler names!

also, change a ptr_mut to ptr where we just read the contents in the
argument marshalling for structs. this has no effect, but it is more
correct.

tests/pointers.rs

@@ -54,7 +54,7 @@ impl pointers::Pointers for WasiCtx {
         println!("input4 {:?}", input4);
 
         // Write ptr value to mutable ptr:
-        input4_ptr_ptr.write_ptr_to_guest(&input2_ptr.as_immut());
+        input4_ptr_ptr.write(&input2_ptr.as_immut());
 
         Ok(())
     }