Process expansions are now being performed as an internal fork within the shell.
Previously, this was done by passing executing a new Ion binary with a given command.
Work is not 100% complete, however. There's some TODOs that label what's not done yet.

- Quoting rules for process expansions has been fixed
- However, we should investigate a performance optimization to eliminate heap allocations there.
- At the moment, as with background shells, stdin within the child of the fork is disabled. Fix this.
- And, we need to remove our pre-process expansion expansions.