Add Math::BigInt and Math::BigFloat methods bnstr(), bestr(), and bdstr(),
as well as corresponding methods nparts(), eparts(), and dparts(). Also add
test files bdstr-mbf.t, bdstr-mbi.t, bestr-mbf.t, bestr-mbi.t, bnstr-mbf.t,
bnstr-mbi.t, bsstr-mbf.t, bsstr-mbi.t, dparts-mbf.t, dparts-mbi.t,
eparts-mbf.t, eparts-mbi.t, nparts-mbf.t, nparts-mbi.t, sparts-mbf.t, and
sparts-mbi.t.Fix documentation errors.