core: Convert TODOs to FIXMEs

Author: brson

src/libcore/cell.rs

@@ -155,10 +155,10 @@
 //! }
 //! ```
 //!
-// TODO: Explain difference between Cell and RefCell
-// TODO: Downsides to interior mutability
-// TODO: Can't be shared between threads. Dynamic borrows
-// TODO: Relationship to Atomic types and RWLock
+// FIXME: Explain difference between Cell and RefCell
+// FIXME: Downsides to interior mutability
+// FIXME: Can't be shared between threads. Dynamic borrows
+// FIXME: Relationship to Atomic types and RWLock
 
 use clone::Clone;
 use cmp::Eq;

src/libcore/lib.rs

@@ -28,7 +28,7 @@
 //!
 //! # How to use the core library
 //!
-// TODO: Fill me in with more detail when the interface settles
+// FIXME: Fill me in with more detail when the interface settles
 //! This library is built on the assumption of a few existing symbols:
 //!
 //! * `memcpy`, `memcmp`, `memset` - These are core memory routines which are