Don't allocate in rt::context::Context::new

[catamorphism]

FIXME says: "Putting main into a ~ so it's a thin pointer and can
// be passed to the spawn function. Another unfortunate
// allocation". Figure out how to not do this.

[emberian]

FIXME still there. I don't see an obvious way to work around this. As it is it's going to be a tiny two-word allocation, and I don't think this is going to be an issue.