feat: map/reduce (#1010)

Signed-off-by: Louis Mandel <[email protected]>

Author: mandel

examples/map-reduce/python-reduce.pdl

@@ -0,0 +1,13 @@
+defs:
+  plus:
+    lang: python
+    code: |
+      import operator
+      result = operator.add
+for:
+  i: [1,2,3,4]
+map:
+  ${i}
+join:
+  as: reduce
+  reduce: ${ plus }

examples/map-reduce/reduce.pdl

@@ -0,0 +1,13 @@
+defs:
+  plus:
+    function:
+      x: number
+      y: number
+    return:
+      ${ x + y }
+for:
+  i: [1,2,3,4]
+map:
+  ${i}
+join:
+  reduce: ${ plus }