Write the core of the header to stream

Property definitions

nitc $ CCompilationUnit :: compile_header_core
	# Write the core of the header to `stream`
	fun compile_header_core(stream: Writer)
	do
		header_c_base.write_to stream
		header_custom.write_to stream
		header_c_types.write_to stream
		header_decl.write_to stream
	end
src/c_tools.nit:66,2--73,4