[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/components/__snapshots__/error_handling.test.ts.snap

@@ -950,7 +950,7 @@ exports[`can catch errors catching error, rethrow, render parent  -- a main comp
     ctx = Object.create(ctx);
     const [k_block1, v_block1, l_block1, c_block1] = prepareList(Object.values(ctx['state'].cps));;
     for (let i1 = 0; i1 < l_block1; i1++) {
-      ctx[\`cp\`] = v_block1[i1];
+      ctx[\`cp\`] = k_block1[i1];
       const key1 = ctx['cp'].id;
       const v1 = ctx['this'];
       const v2 = ctx['cp'];
@@ -1029,7 +1029,7 @@ exports[`can catch errors catching in child makes parent render 1`] = `
     ctx = Object.create(ctx);
     const [k_block1, v_block1, l_block1, c_block1] = prepareList(Object.entries(ctx['this'].elements));;
     for (let i1 = 0; i1 < l_block1; i1++) {
-      ctx[\`elem\`] = v_block1[i1];
+      ctx[\`elem\`] = k_block1[i1];
       const key1 = ctx['elem'][0];
       const v1 = ctx['this'];
       const v2 = ctx['elem'];