fix: shouldn't not stringify static for top level inside slot, but ca…

[underfin]

…n work on themselves children

fix #1281

[yyx990803]

I took another look at this and I think it is better to just bail stringification for all slot content.

Pro: ensures child components can always expect to work with vdom nodes regardless of depth.

Cons: cannot take advantage of stringification inside slots - which seems to be an ok trade-off since the nodes are still hoisted, and it is quite rare for large pieces of static content to be placed inside slots.