Implement FromIterator for SpannedString (#512)

This patch implements FromIterator<SpannedString<T>> for
SpannedString<T> to make it easier to create strings programatically.
We could also use fold directly without extracting the first element,
but that would require an additional allocation.

cursive-core/src/utils/span.rs

@@ -3,6 +3,7 @@
 //! This module defines various structs describing a span of text from a
 //! larger string.
 use std::borrow::Cow;
+use std::iter::FromIterator;
 use unicode_width::UnicodeWidthStr;
 
 /// A string with associated spans.
@@ -254,6 +255,22 @@ impl<T> SpannedString<T> {
     }
 }
 
+impl<T> FromIterator<SpannedString<T>> for SpannedString<T> {
+    fn from_iter<I: IntoIterator<Item = SpannedString<T>>>(
+        iter: I,
+    ) -> SpannedString<T> {
+        let mut iter = iter.into_iter();
+        if let Some(first) = iter.next() {
+            iter.fold(first, |mut acc, s| {
+                acc.append(s);
+                acc
+            })
+        } else {
+            SpannedString::new()
+        }
+    }
+}
+
 impl<'a, T> From<&'a SpannedString<T>> for SpannedStr<'a, T> {
     fn from(other: &'a SpannedString<T>) -> Self {
         SpannedStr::new(&other.source, &other.spans)