New features in this release:

• add ${/} as a companion to ${:} (#90; thanks to @mvdan)
• add Env.Setenv and Env.Getenv (#96; thanks to @twpayne)
• support stdout/stderr args in stdin (#94; thanks to @mvdan)
• expose current subtest in Env (#95; thanks to @frankban)

Also apply fix to lockedfile to fix #91 (thanks to @myitcv)