Avoid unnecessary implicit view on String

Using length instead of size on String to avoid a conversion call.

This dump confirms there is a conversion to StringOps when using size.

    object StringSize {
      val s = "hi"
      println(s.size)
    }

    $ scalac -Xprint:typer StringSize.scala
    [[syntax trees at end of                     typer]] // StringSize.scala
    package <empty> {
      object StringSize extends scala.AnyRef {
        def <init>(): StringSize.type = {
          StringSize.super.<init>();
          ()
        };
        private[this] val s: String = "hi";
        <stable> <accessor> def s: String = StringSize.this.s;
        scala.this.Predef.println(scala.this.Predef.augmentString(StringSize.this.s).size)
      }
    }

Author: janekdb

src/library/scala/collection/immutable/StringLike.scala

@@ -217,8 +217,8 @@ self =>
         pos = thisString.indexOf(separator, prev)
       } while (pos != -1)
 
-      if (prev != thisString.size)
-        res += thisString.substring(prev, thisString.size)
+      if (prev != thisString.length)
+        res += thisString.substring(prev, thisString.length)
 
       val initialResult = res.result()
       pos = initialResult.length