nanshe/util/iters.py: Add `imap` for both Python 2 and 3.
nanshe/util/iters.py: Add `imap` for both Python 2 and 3.

DudLab/nanshe,jakirkham/nanshe,DudLab/nanshe,jakirkham/nanshe,nanshe-org/nanshe,nanshe-org/nanshe