fixes Decoder template adding missing check

[clararod9]

The Decoder template misses a check, generating a R1CS constraint system that admits multiple outputs for the same input. For example, in case it is instantiated with w = 2 then the generated constraint system

out[0] * inp === 0
out[1] * (inp - 1) === 0
out[0] + out[1] === success
success * (success - 1) === 0

admits multiple solutions for the input inp = 1:

• out[0] === 0, out[1] === 1, success = 1
• out[0] === 0, out[1] === 0, success = 0

The proposed solution adds the missing check satisfying the same specification for the template.

• Success := w < inp
• for i in 0..w: out[i] = 1 if i = inp, else out[i]

However, as adding the check increments the number of non-linear constraints, the proposed solution also defines a new template (Decoder_strict) that can be used to optimize the number of non-linear constraints when the only solutions that are valid are the ones where success == 1. This condition appears in the template Multiplexer that represents the main use of Decoder in the circomlib.

The template Decoder_strict satisfies the following specification:

• In case inp < w:
  for i in 0..w: out[i] = 1 if i = inp, else out[i] = 0
• If w >= inp => the sysetm of constraints does not have any solution

The number of constraints after compiling using --O2 of each one of the templates is:

• Decoder(w): 2 * w + 1
• Decoder_strict(w): w + 1
• Multiplexer(wIn, nIn): (wIn + 1) * nIn (same as previous version of Multiplexer)