Merge MappedDeviceMemory into DeviceMemory, make MemoryAlloc reuse the logic

vulkano/src/memory/allocator/mod.rs

@@ -812,6 +812,13 @@ impl StandardMemoryAllocator {
 ///
 /// See also [the `MemoryAllocator` implementation].
 ///
+/// # Mapping behavior
+///
+/// Every time a new `DeviceMemory` block is allocated, it is mapped in full automatically as long
+/// as it resides in host-visible memory. It remains mapped until it is dropped, which only happens
+/// if the allocator is dropped. In other words, all eligeble blocks are persistently mapped, so
+/// you don't need to worry about whether or not your host-visible allocations are host-accessible.
+///
 /// # `DeviceMemory` allocation
 ///
 /// If an allocation is created with the [`MemoryAllocatePreference::Unknown`] option, and the