std: Change assert_eq!() to use {} instead of {:?}

Formatting via reflection has been a little questionable for some time now, and
it's a little unfortunate that one of the standard macros will silently use
reflection when you weren't expecting it. This adds small bits of code bloat to
libraries, as well as not always being necessary. In light of this information,
this commit switches assert_eq!() to using {} in the error message instead of
{:?}.

In updating existing code, there were a few error cases that I encountered:

* It's impossible to define Show for [T, ..N]. I think DST will alleviate this
  because we can define Show for [T].
* A few types here and there just needed a #[deriving(Show)]
* Type parameters needed a Show bound, I often moved this to `assert!(a == b)`
* `Path` doesn't implement `Show`, so assert_eq!() cannot be used on two paths.
  I don't think this is much of a regression though because {:?} on paths looks
  awful (it's a byte array).

Concretely speaking, this shaved 10K off a 656K binary. Not a lot, but sometime
significant for smaller binaries.

src/libsyntax/codemap.rs

@@ -32,13 +32,13 @@ pub trait Pos {
 
 /// A byte offset. Keep this small (currently 32-bits), as AST contains
 /// a lot of them.
-#[deriving(Clone, Eq, Hash, Ord)]
+#[deriving(Clone, Eq, Hash, Ord, Show)]
 pub struct BytePos(u32);
 
 /// A character offset. Because of multibyte utf8 characters, a byte offset
 /// is not equivalent to a character offset. The CodeMap will convert BytePos
 /// values to CharPos values as necessary.
-#[deriving(Eq, Hash, Ord)]
+#[deriving(Eq, Hash, Ord, Show)]
 pub struct CharPos(uint);
 
 // FIXME: Lots of boilerplate in these impls, but so far my attempts to fix
@@ -84,7 +84,7 @@ are *absolute* positions from the beginning of the codemap, not positions
 relative to FileMaps. Methods on the CodeMap can be used to relate spans back
 to the original source.
 */
-#[deriving(Clone, Hash)]
+#[deriving(Clone, Show, Hash)]
 pub struct Span {
     lo: BytePos,
     hi: BytePos,
@@ -160,7 +160,7 @@ pub struct LocWithOpt {
 pub struct FileMapAndLine {fm: @FileMap, line: uint}
 pub struct FileMapAndBytePos {fm: @FileMap, pos: BytePos}
 
-#[deriving(Clone, Hash)]
+#[deriving(Clone, Hash, Show)]
 pub enum MacroFormat {
     // e.g. #[deriving(...)] <item>
     MacroAttribute,
@@ -168,7 +168,7 @@ pub enum MacroFormat {
     MacroBang
 }
 
-#[deriving(Clone, Hash)]
+#[deriving(Clone, Hash, Show)]
 pub struct NameAndSpan {
     name: ~str,
     // the format with which the macro was invoked.
@@ -177,7 +177,7 @@ pub struct NameAndSpan {
 }
 
 /// Extra information for tracking macro expansion of spans
-#[deriving(Hash)]
+#[deriving(Hash, Show)]
 pub struct ExpnInfo {
     call_site: Span,
     callee: NameAndSpan