Expose whether _default_ space for Plan can pin objects or not

[k-sareen]

This is useful if you want to allocate non-moving objects into the default space and pin them instead of having a separate non-moving space.

[qinsoon]

I plan to expose some info about spaces to bindings. But those are mostly for debugging, or for bindings to do optimizations for certain plans.

if you want to allocate non-moving objects into the default space and pin them instead of having a separate non-moving space.

Why do you want to use pin instead of non moving?

[k-sareen]

Pinning needs to be enabled for the ZygoteSpace since we compact into it. And if we already have the pin bit, then we can just use that instead of having a separate non-moving space if the selected plan is Immix-based.

EDIT: This is also useful for JNI critical sections as we can just pin the object instead of deferring or disabling GC