This figures out the size of Blob/File to chose a better arena, potentially
avoiding holding a .large arena (which we have limited) for small data.
This happens to circumvent an allocation bug in std.Io.Writer.Allocating that
can over-reserve memory.
It also removes some page: *Page parameters in favor of *js.Execution which I
think should always be preferred (for no other reason that going from
Execution -> Page is easy, but going from Page -> Execution is more complicated
so we should just always use Execution).
Karl Seguin
8c7706f6a1