[FIX] compiler: fix swapped key/value using t-foreach on Map

In odoo/owl#1352 we added support for using t-foreach on Map objects,
maps should behave the same as objects where keys are available under
the name specified in t-as and values under that same name with the
suffix `_value`. Unfortunately, because the code for objects was written
in a confusing way (values were stored in a variable named `keys` and
vice versa), the implementation for Map was incorrect and keys and
values were swapped.

This commit fixes that and rewrites the code to be less confusing: keys
are extracted from the variables `k_block` and values from `v_block`,
which swaps the behaviour, and the code to prepare a list from an object
now extracts the keys in `keys` and the values in `values`

src/runtime/template_helpers.ts

@@ -75,8 +75,8 @@ function prepareList(collection: unknown): [unknown[], unknown[], number, undefi
       keys = [...(<Iterable<unknown>>collection)];
       values = keys;
     } else {
-      values = Object.keys(collection);
-      keys = Object.values(collection);
+      values = Object.values(collection);
+      keys = Object.keys(collection);
     }
   } else {
     throw new OwlError(`Invalid loop expression: "${collection}" is not iterable`);