Currently, for example, the function declaration symbol member for
checking whether multiplication *does not* overflow is called
`m_bv_smul_ovfl`.  Since we are introducing the upcoming smtlib2 symbols
that check that multpliciation *does* overflow, the not overflow check
symbols are renamed to `m_bv_smul_no_ovfl` etc.

Smtlib2 is being extended to include overflow predicates for bit
vectors (see https://groups.google.com/u/1/g/smt-lib/c/J4D99wT0aKI).
This commit introduces the predicates `bvumulo` and `bvsmulo` that
return `true` if the unsigned multiplication overflows or the signed
multiplication underflows or overflows, respectively.