Add Buffer type field as PhantomData.

[jsatka]

Add PhantomData type to Buffer indicating the type of data the Buffer points to. There is an example in Unused type parameter section of the PhantomData marker docs.

The sole purpose of the type field is to help Rust compiler's type checking, preventing accidentally reading to result array of another type or writing data with different type using CommandQueue's enqueue_ methods. The field is hidden in docs becaue it has size of zero bytes i.e. it doesn't exist at runtime. This hiding in the docs has been the choice e.g. in euclid math tools crate.

If data with different type needs to be written or read into result array of another type however, the type parameter can be explicitly cast with the cast() method, which essentially creates a clone of the Buffer with the new type. Although other methods for achieving the same would exist, such as casting the data to correct type before writing or after reading to result array.

[kenba]

Thank you jsatka, that's a good improvement.
I'll raise an issue to consider adding PhantomData to the other memory objects.