It's not quite clear to me what the intent was here, but I don't believe
this change was correct. We shouldn't block on callbacks within the main
loop.
This fixes #901.
Nested exec expressions like ${execp echo '${execp echo hi}'} are not
evaluated correctly because parsing expressions like these generates and
registers new callbacks but we never wait for these callbacks to
complete before returning the result of the evaluation.
Fix this by re-evaluating expressions after running all pending
callbacks.
