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/rustc/metadata/decoder.rs

@@ -120,7 +120,7 @@ fn class_member_id(d: ebml::doc, cdata: cmd) -> ast::def_id {
 
 fn field_mutability(d: ebml::doc) -> ast::class_mutability {
     // Use maybe_get_doc in case it's a method
-    option::maybe(ebml::maybe_get_doc(d, tag_class_mut),
+    option::with_option(ebml::maybe_get_doc(d, tag_class_mut),
                   ast::class_immutable,
                   {|d|
                   alt ebml::doc_as_u8(d) as char {