core: split into fmt::Show and fmt::String

fmt::Show is for debugging, and can and should be implemented for
all public types. This trait is used with `{:?}` syntax. There still
exists #[derive(Show)].

fmt::String is for types that faithfully be represented as a String.
Because of this, there is no way to derive fmt::String, all
implementations must be purposeful. It is used by the default format
syntax, `{}`.

This will break most instances of `{}`, since that now requires the type
to impl fmt::String. In most cases, replacing `{}` with `{:?}` is the
correct fix. Types that were being printed specifically for users should
receive a fmt::String implementation to fix this.

Part of #20013

[breaking-change]

src/libsyntax/codemap.rs

@@ -488,7 +488,7 @@ impl CodeMap {
         let mut total_extra_bytes = 0;
 
         for mbc in map.multibyte_chars.borrow().iter() {
-            debug!("{}-byte char at {}", mbc.bytes, mbc.pos);
+            debug!("{}-byte char at {:?}", mbc.bytes, mbc.pos);
             if mbc.pos < bpos {
                 // every character is at least one byte, so we only
                 // count the actual extra bytes.
@@ -566,9 +566,9 @@ impl CodeMap {
         let chpos = self.bytepos_to_file_charpos(pos);
         let linebpos = (*f.lines.borrow())[a];
         let linechpos = self.bytepos_to_file_charpos(linebpos);
-        debug!("byte pos {} is on the line at byte pos {}",
+        debug!("byte pos {:?} is on the line at byte pos {:?}",
                pos, linebpos);
-        debug!("char pos {} is on the line at char pos {}",
+        debug!("char pos {:?} is on the line at char pos {:?}",
                chpos, linechpos);
         debug!("byte is on line: {}", line);
         assert!(chpos >= linechpos);