Return result of program from internal execute function

[istathar]

Refactor executeActual to be an silently exposed function returning a value. The signature is

executeActual :: Context τ -> Program τ α -> IO α

This is called by execute and executeWith already; they just discard that value.

This may be the missing link to support proper test machinery for the Program monad. The real test would be if it can be used multiple times in succession in a GHCi session; that would imply we've got the resource cleanup completely solid.

[istathar]

Something's not right:

λ> context <- configure "0" None blankConfig
λ> executeActual context (do { setVerbosityLevel Debug ; debug "what" (packRope "The") })
12:14:30Z (01.851) what = The
λ> executeActual context (do { setVerbosityLevel Debug ; debug "what" (packRope "The") })
<HANGS>

In fact it hangs so badly ^C can't kill it off. Hm.