Add 'transaction.value.is_zero'

  Fixes aiken-lang#67.

Author: KtorZ

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 - [`list.index_of`](https://aiken-lang.github.io/stdlib/aiken/list.html#index_of): For getting a values index in a list.
 - [`transaction.placeholder`](https://aiken-lang.github.io/stdlib/aiken/transaction.html#placeholder): For constructing test transactions.
+- [`transaction.value.is_zero`](https://aiken-lang.github.io/stdlib/aiken/transaction/value.html#is_zero): For checking whether a value is null.
 
 ### Changed
 

lib/aiken/transaction/value.ak

@@ -39,6 +39,11 @@ pub fn zero() -> Value {
   Value { inner: dict.new() }
 }
 
+/// Check is a `Value` is zero. That is, it has no assets and holds no Ada/Lovelace.
+pub fn is_zero(self: Value) -> Bool {
+  self == zero()
+}
+
 /// Construct a `Value` from an asset identifier (i.e. `PolicyId` + `AssetName`)
 /// and a given quantity.
 pub fn from_asset(