[LLHD] Replace entity with module

[maerhart]

• use !hw.inout instead of !llhd.sig to make things work with hw.module (since inout types don't allow nested signals)
• remove the llhd.entity operation and use the hw.module instead. This has two disadvantages: (1) all ports are now inout instead of separated in and out ports, however this was never verified in LLHD anyways and out ports were always more like inout, (2) instance and signal names cannot be verified to be unique within one module anymore
• the HWToLLHD pass is removed because it only converted hw.module to llhd.entity and hw.instance to llhd.inst. We can potentially implement the ModuleLike interface for ProcOp and get rid of llhd.inst as well.
• the function inlining pass does not work yet

[fabianschuiki]

This is awesome 😍 !