macros: init MethodDef.unify_fieldless_variants to false
macros: init MethodDef.unify_fieldless_variants to false

AFAICT this field is irrelevant to us, but it still must be initialized.

vks/num,rust-num/num,rust-num/num-bigint,DonSheddow/num,rust-lang/num,koverstreet/num,rust-num/num,rust-lang/num,DonSheddow/num,rust-num/num-integer,vks/num,koverstreet/num,rust-num/num-integer,rust-num/num-traits,rust-num/num-bigint,rust-num/num-traits