Add missing QF_BV functions

[Robbepop]

The new stevia_bitvec crate and its provided Bitvec implementation is still missing some core QF_BV functions. Namely, bvcomp with proper error handling (not panicking), rotate_left and rotate_right (supported in the upcoming apint version 0.3), and repeat.

For more information visit this page: http://smtlib.cs.uiowa.edu/logics-all.shtml#QF_BV