[MRG] Reduce memory consumption

[trevorstephens]

• Remove any programs from earlier generations that are not part of the current generation
• Make indices an attribute that is calculated on the fly, removing the need to store potentially large arrays of integers within each program

[coveralls]

Coverage decreased (-0.1%) to 98.669% when pulling d3bdac2 on reduce-memory-consumption into d7d6b4c on master.

[coveralls]

Coverage increased (+0.2%) to 98.992% when pulling ac13cee on reduce-memory-consumption into d7d6b4c on master.

[trevorstephens]

Fixes #5 ... at least makes a significant dent in the problem. Bloat can still cause issues, thinking about a fix for that though it is not as trivial.

[coveralls]

Coverage increased (+0.2%) to 98.992% when pulling 763670f on reduce-memory-consumption into d7d6b4c on master.

[coveralls]

Coverage increased (+0.2%) to 98.992% when pulling a948d89 on reduce-memory-consumption into d7d6b4c on master.