Separate executable and library code

[TikhonJelvis]

Based on @istathar's suggestion in #37

[as an aside, and worthy of another issue, in a parallel universe I might suggest putting the library in theta/ and the actual Theta binary in theta-tool/ or something (because otherwise people depending on the library have the binary installed in their local work dir every time), but that's a minor detail and not nearly as impactful as the main change above was. This is mostly just disk space]