compiler.tree.propagation.*: implements downgrading to fixnum for the

compiler.tree.propagation.*: implements downgrading to fixnum for the
mod word, maybe a first step in solving #224

factor_vm::primitive_bignum_mod is changed so that it outputs a fixnum
if the value is small enough. Then the compiler can take advantage of
knowing that expressions like [ >bignum 10 mod ] always result in a
fixnum [-9,9] and inline more arithmetic.