Refactor CallInfo function signatures to new FunctionSignature type

This is used by CallInfo to create a pretty printed function signature that can
be used with completions and other places as well.

crates/ra_ide_api/src/display.rs

@@ -0,0 +1,51 @@
+use super::*;
+use std::fmt::{self, Display};
+
+impl Display for FunctionSignature {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        if let Some(t) = &self.visibility {
+            write!(f, "{} ", t)?;
+        }
+
+        if let Some(name) = &self.name {
+            write!(f, "fn {}", name)?;
+        }
+
+        if !self.generic_parameters.is_empty() {
+            write!(f, "<")?;
+            write_joined(f, &self.generic_parameters, ", ")?;
+            write!(f, ">")?;
+        }
+
+        write!(f, "(")?;
+        write_joined(f, &self.parameters, ", ")?;
+        write!(f, ")")?;
+
+        if let Some(t) = &self.ret_type {
+            write!(f, " -> {}", t)?;
+        }
+
+        if !self.where_predicates.is_empty() {
+            write!(f, "\nwhere ")?;
+            write_joined(f, &self.where_predicates, ",\n      ")?;
+        }
+
+        Ok(())
+    }
+}
+
+fn write_joined<T: Display>(
+    f: &mut fmt::Formatter,
+    items: impl IntoIterator<Item = T>,
+    sep: &str,
+) -> fmt::Result {
+    let mut first = true;
+    for e in items {
+        if !first {
+            write!(f, "{}", sep)?;
+        }
+        first = false;
+        write!(f, "{}", e)?;
+    }
+    Ok(())
+}