Add a recursive object merge strategy and bind it to *

This commit adds a jv_object_merge_recursive function, that performs
recursive object merging, and binds it to multiply when applied to
two objects.

Added docs and tests.

Closes #320

tests/all.test

@@ -779,3 +779,15 @@ null
 map([1,2][0:.])
 [-1, 1, 2, 3, 1000000000000000000]
 [[1], [1], [1,2], [1,2], [1,2]]
+
+{"k": {"a": 1, "b": 2}} * .
+{"k": {"a": 0,"c": 3}}
+{"k": {"a": 0, "b": 2, "c": 3}}
+
+{"k": {"a": 1, "b": 2}, "hello": {"x": 1}} * .
+{"k": {"a": 0,"c": 3}, "hello": 1}
+{"k": {"a": 0, "b": 2, "c": 3}, "hello": 1}
+
+{"k": {"a": 1, "b": 2}, "hello": 1} * .
+{"k": {"a": 0,"c": 3}, "hello": {"x": 1}}
+{"k": {"a": 0, "b": 2, "c": 3}, "hello": {"x": 1}}