Restrict indefinite-length data items

[schomatis]

The standard does not impose any restriction for these but we need a sane value that reflects the limitations of the implementation. Same for nested items.

Additionally any allocation should have a single point of entry where this restrictions is enforced.

This mostly concerns the Deferred type,

cbor-gen/utils.go

Line 123 in 6496743

buf := make([]byte, int(extra)+len(header))

cbor-gen/utils.go

Lines 142 to 144 in 6496743

	for i := 0; i < int(extra); i++ {
	sub := new(Deferred)
	if err := sub.UnmarshalCBOR(br); err != nil {