Validate that there are enough ins/outs for significant privacy

[sneurlax]

See

fusiondart/lib/src/fusion.dart

Lines 1367 to 1370 in 0813241

	// TODO check the components list and see if there are enough inputs/outputs
	// for there to be significant privacy.
	//
	// This is also a TODO in the Python reference: https://github.com/Electron-Cash/Electron-Cash/blob/397f43c848e270a7405795c4643a2805d872675f/electroncash_plugins/fusion/fusion.py#L936-L937

[sneurlax]

Marked wontfix because the Python doesn't do it, either.