Add missing QF_BV functions

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