Changed validate_region function to return PageSpan instead of a tuple. All the code
using validate_region function was updated to use PageSpan as well.

Chunk size of the scheme (`size_of::<ITimerScheme>() == 0`) was being
provided instead of `size_of::<ITimerSpec>()`. This resulted in a kernel
panic (division by zero) when kread was called for this scheme.

Signed-off-by: Anhad Singh <andypython@protonmail.com>

Fixes Raspberry 3B+ DTB parsing (as generated by Qemu)

Signed-off-by: Anhad Singh <andypython@protonmail.com>

Before it was first add_ref'ed by `borrow_frame_enforce_rw_allocated`,
manually and then by `allocated_shared_one_page`.

Now it is only done by `borrow_frame_enforce_rw_allocated` and does not
get unref-ed as take() is called on the returned `RaiiFrame`.

Now the page is manually mapped and an `Allocated` type grant is
constructed (synonymous to `MAP_PRIVATE`). Before by using `allocated_shared_one_page`
an `AllocatedShared` provided grant was constructed (synonymous to
`MAP_SHARED`), which was wrong as the TCB would've not got CoW-ed
after fork(), making the Tcb malformed.

Signed-off-by: Anhad Singh <andypython@protonmail.com>

Signed-off-by: Anhad Singh <andypython@protonmail.com>
Co-authored-by: @4lDO2

Former one is deprecated and apparently only used by QEMU. Latter is used by QEMU as well as others.

* Add documentation and more code comments to `src/context/switch.rs`
* Eliminate a `context.wake.expect(...)` where `context.wake.is_some()`
* Eliminate a TODO item about updating contexts' timestamps in `switch_to`
* Eliminate a dangling `else { continue }` at the end of the loop that iterates
  on contexts

* Mention the need to have `nasm` available on the PATH in the README
* Replace the deprecated `hide_parse_errors` by `show_parse_errors` in `rustfmt.toml`
* Mark unused variables in `src/scheme/proc.rs`