reimplemented+simplified switchable server logging

don't know how well it interacts with `log` (or upcoming tracing), so this may be temporary and/or incomplete. But it works for my application code at the least (as opposed to library code)

server/sourcefile/src/source_mapper.rs

@@ -7,7 +7,7 @@ pub struct SourceNum(usize);
 
 impl Display for SourceNum {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        f.write_str(format!("{}", self.0).as_str())
+        f.write_str(self.0.to_string().as_str())
     }
 }