Refactor shared memory host mappings (#1485)

* Refactor host mapping to use RAII guards

Wrap host-mapping lifetime management in dedicated RAII guards. On Linux `Mmap` owns the `mmap` reservation and calls `munmap` on drop. On Windows `MappedView` owns the mapped view and calls `UnmapViewOfFile`, and `FileMapping` owns the file-mapping HANDLE and calls `CloseHandle`.

`HostMapping` becomes a thin aggregate of these guards instead of carrying raw `(ptr, size, handle)` fields with a manual `Drop` impl, and exposes `ptr()`, `size()`, and `file_mapping_handle()` accessors that the `SharedMemory` trait defaults use.

Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>

* Move stuff around

Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>

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Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>

Author: ludfjig