Feature/tool engine

[swittlich]

• GHDL is now registered via the ToolEngine
• The PreCompileStep now checks whether the top-level entity is a VHDL file, so that it is not executed for Verilog files.
• Cleanups
• ghdl now uses a gen_verilog folder. This folder is located in the build folder and will be included by the standard Yosys toolchain going forward.