boolean vector constructors cannot be const yet

[gnzlbg]

Because const fn does not support if-else branches:

#[inline]
pub /*const*/ fn new($($elem_name: bool),*) -> Self {
    $id($(Self::bool_to_internal($elem_name)),*)
}

#[inline]
/* const */ fn bool_to_internal(x: bool) -> $elem_ty {
    if x  {
        !(0 as $elem_ty)
    } else {
        0 as $elem_ty
    }
}

#[inline]
pub /*const*/ fn splat(value: bool) -> Self {
    let value = Self::bool_to_internal(value);
    $id($({
        #[allow(non_camel_case_types, dead_code)]
        struct $elem_name;
        value
    }),*)
}

EDIT: this issue will make sense as soon as I send the boolean vector PR.

[gnzlbg]

Thanks to @rkruppe's suggestion boolean vectors now have const constructors in #338 .