PHP.hotSwapPHPRuntime: Reuse the FS object instead of copying its contents

[adamziel]

#990 introduced the ability to hot-swap PHP runtimes. Unfortunately, I didn't find an easy way of reusing the existing FS object and had to introduce a recreateMemFs helper that copies the data to the new filesystem.

Perhaps that's unnecessary. Let's explore reusing the old FS object directly.

[adamziel]

Perhaps the same PROXYFS technique as used in #1069 would work here. I'm just worried it would require retaining the entire HEAP of the original PHP instance even after that process is killed.