Configurable option to control stack sizes

[dhil]

It would be useful to have (initial) stack size be a configurable option. The size of async stacks is already a configurable, but we should probably have our own separate option.

[dhil]

This issue, as described, has resolved in a previous patch by @frank-emrich. One useful addition would be to expose these facilities in the C API.

[dhil]

We should also have configurable options to control the stack allocation policy (initial stack pool soze, growth rate, growth technique).