Memory types in VM are described by methods. Each mapped region has
a type, and all pages instantiated get that type on creation.
Individual page types has to be able to change though. This commit
changes the code to use the memory types of the individual pages,
where appropriate, instead of just the higher-level region, in case
it has changed. This is needed to e.g. support future copy-on-write
MAP_PRIVATE mmap modes.