Add function signatures for aliases of built-in functions (#201)

Otherwise, Emacs cannot show helpful signatures, and just uses ARG1 ARG2
instead.

For example, prior to this change, `-drop` was described as:

    -drop is an alias for `nthcdr' in `dash.el'.

    (-drop ARG1 ARG2)

    Return the tail of LIST without the first N items.

dash.el

@@ -469,7 +469,9 @@ See also: `-flatten'"
   (apply 'append lists))
 
 (defalias '-copy 'copy-sequence
-  "Create a shallow copy of LIST.")
+  "Create a shallow copy of LIST.
+
+\(fn LIST)")
 
 (defun -splice (pred fun list)
   "Splice lists generated by FUN in place of elements matching PRED in LIST.
@@ -572,7 +574,9 @@ Alias: `-any'"
   (--last (funcall pred it) list))
 
 (defalias '-first-item 'car
-  "Return the first item of LIST, or nil on an empty list.")
+  "Return the first item of LIST, or nil on an empty list.
+
+\(fn LIST)")
 
 (defun -last-item (list)
   "Return the last item of LIST, or nil on an empty list."
@@ -723,7 +727,9 @@ See also: `-take'"
 (defalias '-drop 'nthcdr
   "Return the tail of LIST without the first N items.
 
-See also: `-drop-last'")
+See also: `-drop-last'
+
+\(fn N LIST)")
 
 (defun -drop-last (n list)
   "Remove the last N items of LIST and return a copy.