rust: Move towards stable Rust

[SlyMarbo]

Currently, the Firefly kernel requires nightly Rust, as we use various unstable features. We should try to migrate to stable Rust where doing so does not cause an unnecessary burden. For example, it is preferable to use the x86-64 interrupt handler ABI as an unstable feature than to rewrite that code to use a stable alternative.

This is the tracking issue for all use of unstable features.

Currently used unstable features:

• abi_x86_interrupt (kernel/interrupts)
• alloc_error_handler (kernel)
• asm_const (kernel/cpu)
• binary_heap_retain (kernel/multitasking)
• bool_to_option (stable, vendor/rust/aml)
• box_syntax (vendor/rust/aml)
• const_btree_new (kernel, kernel/drivers/virtio, kernel/multitasking, kernel/network)
• const_mut_refs (kernel/memory/heap, kernel/memory/virtmem)
• decl_macro (kernel/memory/mmio, vendor/rust/aml)
• type_ascription (vendor/rust/aml)