feat: Add comparative kernel benchmarks

[samuelburnham]

Adds a comparative kernel benchmark rig for lean4 in C++, lean4lean, nanoda, ix-lean, and ix-rust. Run with cd Benchmarks/Kernel && lake exe bench-kernel

Notes:

• ix-lean and ix-rust error when checking Init for now, but they work for decls like Nat.add_comm
• ix-rust includes compilation and typechecking in Ix.CompileM.rsCompileEnvFFI, but for proper measurement the typechecking should be split into a separate function
• Only nanoda and ix-rust support parallelism AFAIK and it is enabled by default for both. To measure everything single threaded pass -j 1 to lake exe bench-kernel