Implement new formatter for 'int'.

This is used for constants and patterns, which can carry negative
   values.

crates/aiken-lang/src/format.rs

@@ -702,7 +702,10 @@ impl<'comments> Formatter<'comments> {
     }
 
     pub fn int<'a>(&mut self, s: &'a str, base: &Base) -> Document<'a> {
-        unimplemented!()
+        match s.chars().next() {
+            Some('-') => Document::Str("-").append(self.uint(&s[1..], base)),
+            _ => self.uint(s, base),
+        }
     }
 
     pub fn uint<'a>(&mut self, s: &'a str, base: &Base) -> Document<'a> {

crates/aiken-lang/src/tests/format.rs

@@ -887,3 +887,21 @@ fn first_class_binop() {
 
     assert_fmt(src, src);
 }
+
+#[test]
+fn int_uint() {
+    let src = indoc! { r#"
+        const i = 42
+
+        const j = -14
+
+        fn foo() {
+          when y is {
+            14 -> -14
+            -42 -> 42
+          }
+        }
+    "#};
+
+    assert_fmt(src, src);
+}