[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/__snapshots__/t_ref.test.ts.snap

@@ -105,7 +105,7 @@ exports[`t-ref refs in a loop 1`] = `
     ctx = Object.create(ctx);
     const [k_block2, v_block2, l_block2, c_block2] = prepareList(ctx['items']);;
     for (let i1 = 0; i1 < l_block2; i1++) {
-      ctx[\`item\`] = v_block2[i1];
+      ctx[\`item\`] = k_block2[i1];
       const key1 = ctx['item'];
       const tKey_1 = ctx['item'];
       const v1 = ctx['item'];