Draft first standard module for ByteArray

examples/aiken_std/lib/aiken/bytearray.ak

@@ -0,0 +1,5 @@
+use aiken/builtins
+
+pub fn slice(bytes: ByteArray, start: Int, end: Int) -> ByteArray {
+  builtins.sliceByteString(start, end, bytes)
+}

examples/aiken_std/lib/aiken/context.ak

@@ -1,8 +0,0 @@
-pub type ScriptContext(purpose) {
-    tx_info: TxInfo,
-    script_purpose: purpose,
-}
-
-pub type TxInfo {
-    idk: Int,
-}