add source line number preservation

For source-level debug. Add --line-directives option, defaulted to
statement. Disabled when used with -C. Explicit non-none value
overrides disabling when -C is present

tests/run.scm

@@ -6,6 +6,7 @@
 (include "reader.scm")
 (include "fmt-c-writer.scm")
 (include "utils.scm")
+(include "line-directives.scm")
 
 ;;; Should be the last in the test suite
 (test-exit)