Improve dispatch to a real scheduler using preemption

[LuernOutOfOrder]

This pull request introduces significant improvements to the kernel's scheduler and context switching mechanisms, adds new documentation, and refactors several core data structures. The most notable changes include a complete rewrite of the scheduler logic (including the introduction of run and blocked queues), the addition of a bitmap utility for efficient priority management, and a major refactor of context save/restore assembly routines. Documentation has also been expanded to describe new and existing data structures and scheduler behavior.

Kernel Scheduler and Task Queues:

• Added a comprehensive new documentation file describing the kernel scheduler, including its run and blocked queues, preemption, scheduling model, and invariants. The run queue now uses a bitmap for fast priority lookup, and the blocked queue is implemented as a sorted indexed linked list.
• Introduced new configuration constants for task priorities and queue sizes in src/config.rs, and updated documentation to reflect these changes. [1] [2]

Data Structures and Utilities:

• Added a new Bitmap struct in src/primitives/bitmap.rs to efficiently manage and query task priorities in the run queue.
• Documented the new IndexedLinkedList data structure and its invariants in Documentation/kernel/data_structure.md. [1] [2]

Context Switching and Trap Handling:

• Refactored RISC-V context switch assembly routines: removed saving/restoring of mstatus in the task context, rewrote save_context/restore_context to handle privilege and interrupt bits directly, and introduced trap_save_context/trap_restore_context for trap entry/exit. [1] [2] [3] [4] [5] [6] [7] [8]
• Updated the TaskContext struct in Rust to remove the mstatus field and adjust offsets accordingly. [1] [2]

Miscellaneous and Documentation:

• Increased RAM and ROM sizes in the linker script for test mode.
• Updated kernel version to 0.4.5 in both Cargo.toml and src/info.rs. [1] [2]
• Enabled the stmt_expr_attributes nightly feature in src/main.rs.

These changes collectively modernize the kernel's scheduling and context switching logic, improve documentation, and lay groundwork for further extensibility and debugging.