runtime: allocate map element zero values for reflect-created types on demand
Preallocating them in reflect means that
(1) if you say _ = PtrTo(ArrayOf(
1000000000, reflect.TypeOf(byte(0)))), you just allocated 1GB of data
(2) if you say it again, that's *another* GB of data.
The only use of t.zero in the runtime is for map elements.
Delay the allocation until the creation of a map with that element type,
and share the zeros.
The one downside of the shared zero is that it's not garbage collected,
but it's also never written, so the OS should be able to handle it fairly
efficiently.
Change-Id: I56b098a091abf3ac0945de28ebef9a6c08e76614
Reviewed-on: https://go-review.googlesource.com/10111
Reviewed-by: Keith Randall <khr@golang.org>