[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_set.test.ts.snap

@@ -24,11 +24,10 @@ exports[`t-set slot setted value (with t-set) not accessible with t-esc 2`] = `
 ) {
   let { text, createBlock, list, multi, html, toggler, component } = bdom;
   let { withDefault, getTemplate, prepareList, withKey, zero, call, callSlot, capture, isBoundary, shallowEqual, setContextValue, toNumber, safeOutput } = helpers;
-  let assign = Object.assign;
   
   let block1 = createBlock(\`<div><p><block-text-0/></p><block-child-0/><p><block-text-1/></p></div>\`);
   
-  const slot3 = ctx => (node, key) => {
+  const slot2 = (ctx, node, key) => {
     setContextValue(ctx, \\"iter\\", 'inCall');
   }
   
@@ -37,8 +36,8 @@ exports[`t-set slot setted value (with t-set) not accessible with t-esc 2`] = `
     ctx[isBoundary] = 1
     setContextValue(ctx, \\"iter\\", 'source');
     let d1 = ctx['iter'];
-    const ctx2 = capture(ctx);
-    let b2 = assign(component(\`Childcomp\`, {}, key + \`__1\`, node, ctx), {slots: {'default': slot3(ctx2)}});
+    const ctx1 = capture(ctx);
+    let b2 = component(\`Childcomp\`, {slots: {'default': {__render: slot2, __ctx: ctx1}}}, key + \`__3\`, node, ctx);
     let d2 = ctx['iter'];
     return block1([d1, d2], [b2]);
   }