add is_empty api

Author: Yu-zh

string/view.mbt

@@ -447,3 +447,9 @@ pub fn StringView::iter(self : StringView) -> Iter[Char] {
     }
   })
 }
+
+///|
+/// Returns true if the view is empty.
+pub fn StringView::is_empty(self : StringView) -> Bool {
+  self.start == self.end
+}

string/view_test.mbt

@@ -514,3 +514,14 @@ test "panic stringview negative index3" {
   let _ = str[-1:-2]
 
 }
+
+test "is_empty" {
+  let str = "hello"
+  let view = str[1:4]
+  inspect!(view.is_empty(), content="false")
+  let view = str[0:0]
+  inspect!(view.is_empty(), content="true")
+  let str = ""
+  let view = str[:]
+  inspect!(view.is_empty(), content="true")
+}