extmod/vfs_rom: Add VfsRom filesystem object.

This commit defines a new ROMFS filesystem for storing read-only files that
can be memory mapped, and a new VfsRom driver.  Files opened from this
filesystem support the buffer protocol.  This allows naturally getting the
memory-mapped address of the file using:
- memoryview(file)
- uctypes.addressof(file)

Furthermore, if these files are .mpy files then their content can be
referenced in-place when importing.  Such imports take up a lot less RAM
than importing from a normal filesystem.  This is essentially dynamically
frozen .mpy files, building on the revamped v6 .mpy file format.

Signed-off-by: Damien George <damien@micropython.org>

extmod/modvfs.c

@@ -32,6 +32,7 @@
 #include "extmod/vfs_fat.h"
 #include "extmod/vfs_lfs.h"
 #include "extmod/vfs_posix.h"
+#include "extmod/vfs_rom.h"
 
 #if !MICROPY_VFS
 #error "MICROPY_PY_VFS requires MICROPY_VFS"
@@ -51,6 +52,9 @@ static const mp_rom_map_elem_t vfs_module_globals_table[] = {
     #if MICROPY_VFS_LFS2
     { MP_ROM_QSTR(MP_QSTR_VfsLfs2), MP_ROM_PTR(&mp_type_vfs_lfs2) },
     #endif
+    #if MICROPY_VFS_ROM
+    { MP_ROM_QSTR(MP_QSTR_VfsRom), MP_ROM_PTR(&mp_type_vfs_rom) },
+    #endif
     #if MICROPY_VFS_POSIX
     { MP_ROM_QSTR(MP_QSTR_VfsPosix), MP_ROM_PTR(&mp_type_vfs_posix) },
     #endif