libutils: Mmio + refactoring

[Orycterope]

Splitting up the io module in libutils in three modules:

• mod.rs: Contains the Io trait, and ReadOnly/WriteOnly wrappers,
• pio.rs: Contains the Port IO implementation for x86,
• mmio.rs: New module, contains interface to a Memory Mapped IO.

[todo]

Mmio<T> UnsafeCell

Figure out if Mmio<T> should implement UnsafeCell. Does this mean that, just like atomic, write can take self by const reference only ? But is a Mmio<T> actually atomic ?
Forward all these questions to @roblabla.
Note:
see [volatile cell][https://docs.rs/vcell/0.1.0/src/vcell/lib.rs.html#18-20] and [this discussion][https://github.com/rust-lang/unsafe-code-guidelines/issues/33]

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https://github.com/roblabla42/KFS/blob/06a870e091c8fb94449591a18f43acf4c029ab5e/libutils/src/io/mmio.rs#L42-L52

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This comment was generated by todo based on a todo comment in 06a870e in #198. cc @Orycterope.

[todo]

Mmio<T> UnsafeCell

Figure out if Mmio<T> should implement UnsafeCell. Does this mean that, just like atomic, write can take self by const reference only ? But is a Mmio<T> actually atomic ?
Forward all these questions to @roblabla.
Note:
see volatile cell and this discussion

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https://github.com/roblabla42/KFS/blob/c0c377d444ebf19b1143a9b835b4ff9cbf44f036/libutils/src/io/mmio.rs#L42-L52

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This comment was generated by todo based on a todo comment in c0c377d in #198. cc @Orycterope.

[roblabla]

LGTM. Memio definitely needs an UnsafeCell but we can deal with this later