Cache models

[richfitz]

If the code has not changed, we could get away with retrieving the model from a cache. This would make using odin::odin within a function more reasonable.

[richfitz]

See #62 and #63

[richfitz]

This probably requires a preprocessing step to pull out the config lines because a hash would need to include the hash of the includes and need to deal with the basename (to determine what files to compare with)

[richfitz]

See also jackolney/baldr#10

[richfitz]

Issues to deal with here:

• pool of models
• expire old models and unload the dll to avoid hitting limit (default is 100, tunable to 1000 on startup by setting R_MAX_NUM_DLLS, and only if the number of available fds is high enough)
• when expiring models need to remove and gc and pointers created by each generator