Move beta-related functionality from Clay to OSL

A significant amount of theoretical work on polyhedral tools is written in terms of beta-prefixes and beta-vectors, while the only tool that seems to have this concept implemented is Clay. It seems reasonable for me to introduce an entity osl_beta and move multiple clay_beta_* functions to OSL so that other tools can use it. For example, finding a statement by its beta is rather useful functionality. For example, Candl expects statements sorted by beta-vectors, but can not do it itself without duplicating Clay functionality. Beta is a scop-related notion, rather than Clay-specific. Introducing more functionality to osl may make it heavier, but I don't think it will make reading/writing any slower since betas are extracted from osl_* structures upon request.

Start by locating the existing clay_beta_* functions and the osl_* structures that expose related data. Review how beta-prefixes and beta-vectors are represented and extracted; done should mean beta functionality is available through an osl_beta entity for other tools without duplicating Clay-specific code.