feat(aiken_stdlib): add script context type

examples/aiken_std/lib/aiken/bytearray.ak

@@ -9,7 +9,7 @@ pub fn length(bytes: ByteArray) -> Int {
 }
 
 pub fn is_empty(bytes: ByteArray) -> Bool {
-  builtin.emptyByteArray(bytes)
+  length(bytes) == 0
 }
 
 pub fn concat(left front: ByteArray, right back: ByteArray) -> ByteArray {

examples/aiken_std/lib/aiken/context.ak

@@ -0,0 +1,17 @@
+pub type ScriptContext(purpose) {
+  transaction: Transaction,
+  purpose: purpose,
+}
+
+pub type Transaction {
+  inputs: List(Nil),
+  outputs: List(Nil),
+  fee: Nil,
+  mint: Nil,
+  certificates: List(Nil),
+  withdrawals: List(Nil),
+  validity_range: Nil,
+  extra_signatories: Nil,
+  datums: List(Nil),
+  id: ByteArray,
+}

examples/aiken_std/lib/aiken/context/mint.ak

@@ -0,0 +1,3 @@
+pub type Mint {
+  currency_symbol: ByteArray,
+}

examples/aiken_std/validators/always_true.ak

@@ -1,3 +1,6 @@
-pub fn spend(d: Nil, r: Nil, ctx: Nil) -> Bool {
+use aiken/context.{ScriptContext}
+use aiken/context/mint.{Mint}
+
+pub fn mint(d: Nil, r: Nil, ctx: ScriptContext(Mint)) -> Bool {
   True
 }