use a statically-allocated stack for Context::new benchmarking
use a statically-allocated stack for Context::new benchmarking

Now it's an actual context creation benchmark, instead of benchmarking
our stack creation, which is swappable anyway.

QuiltOS/libfringe,QuiltOS/libfringe,whitequark/libfringe,RustOS-Fork-Holding-Ground/libfringe,RustOS-Fork-Holding-Ground/libfringe,whitequark/libfringe