Add span.as_token() and span.new_ident() (#279)

Add From<Span> for all tokens
diff --git a/src/token.rs b/src/token.rs
index f05968b..d3511c4 100644
--- a/src/token.rs
+++ b/src/token.rs
@@ -181,6 +181,12 @@
                 Some(concat!("`", $s, "`"))
             }
         }
+
+        impl From<Span> for $name {
+            fn from(span: Span) -> Self {
+                $name([span; $len])
+            }
+        }
     }
 }
 
@@ -242,6 +248,12 @@
                 Some(concat!("`", $s, "`"))
             }
         }
+
+        impl From<Span> for $name {
+            fn from(span: Span) -> Self {
+                $name(span)
+            }
+        }
     }
 }
 
@@ -298,6 +310,12 @@
                 parsing::delim($s, tokens, $name, f)
             }
         }
+
+        impl From<Span> for $name {
+            fn from(span: Span) -> Self {
+                $name(span)
+            }
+        }
     }
 }