We can now elaborate
https://gist.github.com/gebner/439273deee592603190d4f8b4447295b
in 1.6 secs and using less than 500Kb of stack space.
It was takins 44 secs and 5Mb before this commit.
Two modifications:
1) Use pre_monad.seq instead of pre_monad.and_then.
They have the same implementation, but seq is not marked as [inline].
2) Modify how we concatenate the tactics in a begin...end block.
Before: (((a_1 ++ a_2) ++ a_3) ++ a_4)
After: ((a_1 ++ a_2) ++ (a_3 ++ a_4))
