[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`

tests/compiler/t_foreach.test.ts

@@ -110,7 +110,7 @@ describe("t-foreach", () => {
       <t t-foreach="value" t-as="item" t-key="item_index">
         [<t t-esc="item_index"/>: <t t-esc="item"/> <t t-esc="item_value"/>]
       </t>`;
-    const expected = ` [0: 1 a]  [1: 2 b]  [2: 3 c] `;
+    const expected = ` [0: a 1]  [1: b 2]  [2: c 3] `;
     const context = {
       value: new Map([
         ["a", 1],