Remove legacy LLHD crate codegen and dependency

[fabianschuiki]

#234 implements LLHD codegen through the CIRCT library. This allows us to stop emitting the legacy IR in src/svlog/codegen.rs, and possibly strip out the dependency on the llhd crate entirely.

Todo

• Merge Link against CIRCT and use it for codegen #234
• Remove codegen through LLHD crate from src/svlog/codegen.rs; use CIRCT exclusively
• Remove the llhd crate as a dependency (at least from the svlog part, but should also be possible on the vhdl side)