feat({hw/emu}): Add asynchronous kernel option

[clabby]

Overview

Adds a new option to use an asynchronous Kernel in the StEmu.

Also a few periphery changes:

• REG_* constants are now usize, better ergonomics given that they're usually used to index.
• JAL, JALR, AUIPC now use wrapping_add.

[codecov]

Codecov Report

❌ Patch coverage is 50.66667% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.0%. Comparing base (8ac0e39) to head (3d9394d).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/emu/src/st/mod.rs	46.6%	24 Missing ⚠️
crates/hw/src/kernel.rs	0.0%	10 Missing ⚠️
crates/emu/src/test_utils.rs	57.1%	3 Missing ⚠️

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