add function to create a LineGap.t from a string (although, for performance reasons, it's best to use this function when string is shorter than 1024 chars)

src/line_gap.sml

@@ -10,11 +10,14 @@ sig
     , rightLines: int vector list
     }
 
-  val delete: int * int * t -> t
   val empty: t
-  val insert: int * string * t -> t
+
+  val fromString: string -> t
   val toString: t -> string
 
+  val delete: int * int * t -> t
+  val insert: int * string * t -> t
+
   (* for testing *)
   val verifyIndex: t -> unit
   val verifyLines: t -> unit
@@ -75,6 +78,15 @@ struct
     , rightLines = []
     }
 
+  fun fromString str =
+    { idx = 0
+    , leftStrings = []
+    , rightStrings = [str]
+    , line = 0
+    , leftLines = []
+    , rightLines = [countLineBreaks str]
+    }
+
   local
     fun helpToString (acc, input) =
       case input of