Remove (lots of) dead code

Found with https://github.com/est31/warnalyzer.

Dubious changes:
- Is anyone else using rustc_apfloat? I feel weird completely deleting
  x87 support.
- Maybe some of the dead code in rustc_data_structures, in case someone
  wants to use it in the future?
- Don't change rustc_serialize

  I plan to scrap most of the json module in the near future (see
  https://github.com/rust-lang/compiler-team/issues/418) and fixing the
  tests needed more work than I expected.

TODO: check if any of the comments on the deleted code should be kept.

compiler/rustc_errors/src/diagnostic.rs

@@ -69,10 +69,6 @@ impl DiagnosticStyledString {
     pub fn highlighted<S: Into<String>>(t: S) -> DiagnosticStyledString {
         DiagnosticStyledString(vec![StringPart::Highlighted(t.into())])
     }
-
-    pub fn content(&self) -> String {
-        self.0.iter().map(|x| x.content()).collect::<String>()
-    }
 }
 
 #[derive(Debug, PartialEq, Eq)]
@@ -81,14 +77,6 @@ pub enum StringPart {
     Highlighted(String),
 }
 
-impl StringPart {
-    pub fn content(&self) -> &str {
-        match self {
-            &StringPart::Normal(ref s) | &StringPart::Highlighted(ref s) => s,
-        }
-    }
-}
-
 impl Diagnostic {
     pub fn new(level: Level, message: &str) -> Self {
         Diagnostic::new_with_code(level, None, message)
@@ -156,7 +144,7 @@ impl Diagnostic {
         self
     }
 
-    pub fn note_expected_found(
+    crate fn note_expected_found(
         &mut self,
         expected_label: &dyn fmt::Display,
         expected: DiagnosticStyledString,
@@ -166,7 +154,7 @@ impl Diagnostic {
         self.note_expected_found_extra(expected_label, expected, found_label, found, &"", &"")
     }
 
-    pub fn note_unsuccessful_coercion(
+    crate fn note_unsuccessful_coercion(
         &mut self,
         expected: DiagnosticStyledString,
         found: DiagnosticStyledString,
@@ -256,33 +244,33 @@ impl Diagnostic {
 
     /// Prints the span with a note above it.
     /// This is like [`Diagnostic::note()`], but it gets its own span.
-    pub fn span_note<S: Into<MultiSpan>>(&mut self, sp: S, msg: &str) -> &mut Self {
+    crate fn span_note<S: Into<MultiSpan>>(&mut self, sp: S, msg: &str) -> &mut Self {
         self.sub(Level::Note, msg, sp.into(), None);
         self
     }
 
     /// Add a warning attached to this diagnostic.
-    pub fn warn(&mut self, msg: &str) -> &mut Self {
+    crate fn warn(&mut self, msg: &str) -> &mut Self {
         self.sub(Level::Warning, msg, MultiSpan::new(), None);
         self
     }
 
     /// Prints the span with a warning above it.
     /// This is like [`Diagnostic::warn()`], but it gets its own span.
-    pub fn span_warn<S: Into<MultiSpan>>(&mut self, sp: S, msg: &str) -> &mut Self {
+    crate fn span_warn<S: Into<MultiSpan>>(&mut self, sp: S, msg: &str) -> &mut Self {
         self.sub(Level::Warning, msg, sp.into(), None);
         self
     }
 
     /// Add a help message attached to this diagnostic.
-    pub fn help(&mut self, msg: &str) -> &mut Self {
+    crate fn help(&mut self, msg: &str) -> &mut Self {
         self.sub(Level::Help, msg, MultiSpan::new(), None);
         self
     }
 
     /// Prints the span with some help above it.
     /// This is like [`Diagnostic::help()`], but it gets its own span.
-    pub fn span_help<S: Into<MultiSpan>>(&mut self, sp: S, msg: &str) -> &mut Self {
+    crate fn span_help<S: Into<MultiSpan>>(&mut self, sp: S, msg: &str) -> &mut Self {
         self.sub(Level::Help, msg, sp.into(), None);
         self
     }
@@ -311,36 +299,6 @@ impl Diagnostic {
         self
     }
 
-    /// Show multiple suggestions that have multiple parts.
-    /// See also [`Diagnostic::multipart_suggestion()`].
-    pub fn multipart_suggestions(
-        &mut self,
-        msg: &str,
-        suggestions: Vec<Vec<(Span, String)>>,
-        applicability: Applicability,
-    ) -> &mut Self {
-        assert!(!suggestions.is_empty());
-        for s in &suggestions {
-            assert!(!s.is_empty());
-        }
-        self.suggestions.push(CodeSuggestion {
-            substitutions: suggestions
-                .into_iter()
-                .map(|suggestion| Substitution {
-                    parts: suggestion
-                        .into_iter()
-                        .map(|(span, snippet)| SubstitutionPart { snippet, span })
-                        .collect(),
-                })
-                .collect(),
-            msg: msg.to_owned(),
-            style: SuggestionStyle::ShowCode,
-            applicability,
-            tool_metadata: Default::default(),
-        });
-        self
-    }
-
     /// Prints out a message with for a multipart suggestion without showing the suggested code.
     ///
     /// This is intended to be used for suggestions that are obvious in what the changes need to
@@ -567,7 +525,7 @@ impl Diagnostic {
         self.code.clone()
     }
 
-    pub fn set_primary_message<M: Into<String>>(&mut self, msg: M) -> &mut Self {
+    crate fn set_primary_message<M: Into<String>>(&mut self, msg: M) -> &mut Self {
         self.message[0] = (msg.into(), Style::NoStyle);
         self
     }