[IMP] slots: via prop 'slots'

The slot inner working has been reworked. A prop "slots" is now passed
explicitely to the component. It looks like

{ slotName_1: slotInfo_1, ..., slotName_m: slotInfo_m }

with the objects slotInfo_i with mandatory keys "__render", "__ctx",
and optional key "__scope" and possibly others.

Here is how a slotInfo object can be created:
A slotInfo object is normally created by setting in a template something
like

<div>
    <t t-set-slot="foo" t-set-scope="scope" param_1="var" param_2="3">
        content
        <t t-esc="scope.bool"/>
        <t t-esc="scope.num"/>
    </t>
</div>

and it will be used somewhere like

<div>
    <t t-esc="props.slots.foo.param_1"/>
    <t t-slot="foo" bool="other_var" num="5">
</div>

In the above example, the function "__render" produces the block dom
element for the content of the t-set-slot.
The context "__ctx" will have a key "scope" with value { bool: ..., num: 5 }
and "__scope" will be set to "scope".

tests/components/__snapshots__/t_foreach.test.ts.snap

@@ -54,9 +54,9 @@ exports[`list of components crash on duplicate key in dev mode 1`] = `
       let key1 = 'child';
       if (keys1.has(key1)) { throw new Error(\`Got duplicate key in t-foreach: \${key1}\`)}
       keys1.add(key1);
-      const props2 = {}
-      helpers.validateProps(\`Child\`, props2, ctx)
-      c_block1[i1] = withKey(component(\`Child\`, props2, key + \`__1__\${key1}\`, node, ctx), key1);
+      const props1 = {}
+      helpers.validateProps(\`Child\`, props1, ctx)
+      c_block1[i1] = withKey(component(\`Child\`, props1, key + \`__2__\${key1}\`, node, ctx), key1);
     }
     return list(c_block1);
   }