Optimize appendString: bulk array copy via String.getChars

Replace char-by-char appendUnsafeC loop with String.getChars bulk copy
directly into CharBuilder's backing array. This eliminates per-character
method call overhead and allows the JVM/native runtime to use optimized
memory copy intrinsics.

Upstream: He-Pin/sjsonnet@5e9686cd (appendString portion)

Author: He-Pin

sjsonnet/src/sjsonnet/BaseCharRenderer.scala

@@ -179,13 +179,15 @@ class BaseCharRenderer[T <: upickle.core.CharOps.Output](
     }
   }
 
+  /**
+   * Bulk-copy String chars into CharBuilder's backing array. Safe because ensureLength(len)
+   * guarantees capacity for currentLength + len.
+   */
   protected def appendString(s: String): Unit = {
     val len = s.length
-    var i = 0
     elemBuilder.ensureLength(len)
-    while (i < len) {
-      elemBuilder.appendUnsafeC(s.charAt(i))
-      i += 1
-    }
+    val pos = elemBuilder.getLength
+    s.getChars(0, len, elemBuilder.arr, pos)
+    elemBuilder.length = pos + len
   }
 }

sjsonnet/test/src/sjsonnet/StdObjectRemoveKeyTests.scala

@@ -19,18 +19,18 @@ object StdObjectRemoveKeyTests extends TestSuite {
 
     test("re-adding removed key via object inheritance") {
       eval("""std.objectRemoveKey({foo: "bar"}, "foo") + {foo: "bar2"}""") ==>
-        ujson.Obj("foo" -> "bar2")
+      ujson.Obj("foo" -> "bar2")
 
       eval("""std.objectRemoveKey({a: 1, b: 2}, "a") + {a: 3}""") ==>
-        ujson.Obj("b" -> 2, "a" -> 3)
+      ujson.Obj("b" -> 2, "a" -> 3)
 
       eval("""std.objectRemoveKey({a: 1, b: 2, c: 3}, "b") + {b: 99, d: 4}""") ==>
-        ujson.Obj("a" -> 1, "c" -> 3, "b" -> 99, "d" -> 4)
+      ujson.Obj("a" -> 1, "c" -> 3, "b" -> 99, "d" -> 4)
     }
 
     test("double removal then re-add") {
       eval(
-        """std.objectRemoveKey(std.objectRemoveKey({a: 1, b: 2, c: 3}, "a"), "b") + {a: 10}"""
+        """std.objeceKey(std.objectRemoveKey({a: 1, b: 2, c: 3}, "a"), "b") + {a: 10}"""
       ) ==> ujson.Obj("c" -> 3, "a" -> 10)
     }
 
@@ -46,18 +46,18 @@ object StdObjectRemoveKeyTests extends TestSuite {
 
     test("external inheritance preserved after removal") {
       eval("{a: 1} + std.objectRemoveKey({b: super.a}, 'a')") ==>
-        ujson.Obj("a" -> 1, "b" -> 1)
+      ujson.Obj("a" -> 1, "b" -> 1)
     }
 
     test("LHS key preserved when RHS removes it") {
       eval("""{a: 1} + std.objectRemoveKey({a: 2}, "a")""") ==>
-        ujson.Obj("a" -> 1)
+      ujson.Obj("a" -> 1)
 
       eval("""{a: 1} + std.objectRemoveKey({a: 2, b: 3}, "a")""") ==>
-        ujson.Obj("a" -> 1, "b" -> 3)
+      ujson.Obj("a" -> 1, "b" -> 3)
 
       eval("""{a: 10} + std.objectRemoveKey({a: 1} + {b: super.a}, "a")""") ==>
-        ujson.Obj("a" -> 10, "b" -> 1)
+      ujson.Obj("a" -> 10, "b" -> 1)
     }
 
     test("containsKey and objectFields reflect re-added key") {