[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".

src/component/component_node.ts

@@ -87,7 +87,6 @@ export class ComponentNode<T extends typeof Component = typeof Component>
   level: number;
   childEnv: Env;
   children: { [key: string]: ComponentNode } = Object.create(null);
-  slots: any = {};
   refs: any = {};
 
   willStart: LifecycleHook[] = [];