feat: add length and concat to aiken/bytearray

examples/aiken_std/lib/aiken/bytearray.ak

@@ -1,5 +1,13 @@
-use aiken/builtins
+use aiken/builtin
 
 pub fn slice(bytes: ByteArray, start: Int, end: Int) -> ByteArray {
-  builtins.sliceByteString(start, end, bytes)
+  builtin.sliceByteArray(start, end, bytes)
+}
+
+pub fn length(bytes: ByteArray) -> Int {
+  builtin.lengthOfByteArray(bytes)
+}
+
+pub fn concat(left front: ByteArray, right back: ByteArray) -> ByteArray {
+  builtin.appendByteArray(front, back)
 }

examples/aiken_std/validators/always_true.ak

@@ -1,3 +1,3 @@
-pub fn spend() -> Bool {
+pub fn spend(d: Nil, r: Nil, ctx: Nil) -> Bool {
   True
 }