Put the `bUUID` class in the `blender` namespace, instead of the
`blender::bke` namespace.
As a result, some C++ code now correctly uses the C++ class, where
previously it would use the C struct and use implicit casting where
necessary. As a result, support for initializer lists had to be
explicitly coded and in another place an explicit `::bUUID` was
necessary to avoid ambiguity.
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