Rename some core::option functions

from_maybe => get_with_default
maybe => with_option
may => with_option_do

I know these names are kind of ridiculous, but it's the best I could think of.
Feel free to bikeshed. Closes #2081

src/libcore/task.rs

@@ -498,7 +498,7 @@ fn spawn_raw(opts: task_opts, +f: fn~()) unsafe {
       }
     };
 
-    option::may(opts.notify_chan) {|c|
+    option::with_option_do(opts.notify_chan) {|c|
         // FIXME (1087): Would like to do notification in Rust
         rustrt::rust_task_config_notify(new_task, c);
     }