collector: Allocate space on heap for chunks

[bkylerussell]

Nicer for embedded devices which may have smaller stack limitations.

[xrmx]

Thanks for the patch, have you hit this in practice? If so on which hardware?

[bkylerussell]

Yes, I hit this trying to run bootchart on a board where we limit stack size to 128K, which CHUNK_SIZE meets. I was running on an ARM platform.