Small fixes, mostly around return

[jpco]

Fixes #145.

• eval is noreturn, so @ {eval return 1; return 2} appropriately returns 1.
• %eval-print is noreturn, so es -xc 'return 99' behaves like es -c 'return 99' does.
• Non-primitive %readfrom and %writeto are noreturn, so return <{true} behaves consistently in both primitive and non-primitive forms.
• share/status.es sets $status based on return exceptions, and keeps %dispatch noreturn.

Not so related to return: true and false don't include their arguments in their return value, such that true blah returns 0 instead of 0 blah, which is falsey.