Currently we hardcode u64 type for shadow everywhere
and do lots of uptr<->u64* casts. It makes it hard to
change u64 to another type (e.g. u32) and makes it easy
to introduce bugs.
Introduce RawShadow type and use it in MemToShadow, ShadowToMem,
IsShadowMem and throughout the code base as u64 replacement.
This makes it possible to change u64 to something else in future
and generally improves static typing.
Depends on D107481.