split up the support of external compilers into a dedicated file

Citadelle/src/compiler/Generator_dynamic_concurrent.thy

Citadelle/src/compiler/Generator_dynamic_export_testing.thy

Citadelle/src/compiler/Generator_dynamic_sequential.thy

Citadelle/src/compiler/Generator_static.thy

Citadelle/src/compiler/Rail.thy

Citadelle/src/compiler/meta/Printer_init.thy

Citadelle/src/compiler_generic/document/Rail.thy

...delle/src/compiler_generic/meta_isabelle/Printer_init.thy

...ic/toy_example/embedding/Generator_dynamic_sequential.thy