Rust changes for v6.10

The most notable change is the drop of the 'alloc' in-tree fork. This
is nicely reflected in the diffstat as a ~10k lines drop. In turn, this
makes the version upgrades way simpler and smaller in the future, e.g.
the latest one in commit 56f64b370612 ("rust: upgrade to Rust 1.78.0").

More importantly, this increases the chances that a newer compiler
version just works, which in turn means supporting several compiler
versions is easier now. Thus we will look into finally setting a minimum
version in the near future.

Toolchain and infrastructure:

 - Upgrade to Rust 1.78.0.

   This time around, due to how the kernel and Rust schedules have
   aligned, there are two upgrades in fact. These allow us to remove one
   more unstable feature ('offset_of') from the list, among other
   improvements.

 - Drop 'alloc' in-tree fork of the standard library crate, which means
   all the unstable features used by 'alloc' (~30 language ones, ~60
   library ones) are not a concern anymore.

 - Support DWARFv5 via the '-Zdwarf-version' flag.

 - Support zlib and zstd debuginfo compression via the
   '-Zdebuginfo-compression' flag.

'kernel' crate:

 - Support allocation flags ('GFP_*'), particularly in 'Box' (via
   'BoxExt'), 'Vec' (via 'VecExt'), 'Arc' and 'UniqueArc', as well as in
   the 'init' module APIs.

 - Remove usage of the 'allocator_api' unstable feature.

 - Remove 'try_' prefix in allocation APIs' names.

 - Add 'VecExt' (an extension trait) to be able to drop the 'alloc'
   fork.

 - Add the '{make,to}_{upper,lower}case()' methods to 'CStr'/'CString'.

 - Add the 'as_ptr' method to 'ThisModule'.

 - Add the 'from_raw' method to 'ArcBorrow'.

 - Add the 'into_unique_or_drop' method to 'Arc'.

 - Display column number in the 'dbg!' macro output by applying the
   equivalent change done to the standard library one.

 - Migrate 'Work' to '#[pin_data]' thanks to the changes in the 'macros'
   crate, which allows to remove an unsafe call in its 'new' associated
   function.

 - Prevent namespacing issues when using the '[try_][pin_]init!' macros
   by changing the generated name of guard variables.

 - Make the 'get' method in 'Opaque' const.

 - Implement the 'Default' trait for 'LockClassKey'.

 - Remove unneeded 'kernel::prelude' imports from doctests.

 - Remove redundant imports.

'macros' crate:

 - Add 'decl_generics' to 'parse_generics()' to support default values,
   and use that to allow them in '#[pin_data]'.

Helpers:

 - Trivial English grammar fix.

Documentation:

 - Add section on Rust Kselftests to the "Testing" document.

 - Expand the "Abstractions vs. bindings" section of the "General
   Information" document.
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