fix: mimc is compliant with ethereum

[ThomasPiellard]

Fixes #124 .

Example:

var data [3]fr.Element
	data[0].SetString("10909369219534740878285360918369814291778422174980871969149168794639722256599")
	data[1].SetString("3811523387212735178398974960485340561880938762308498768570292593755555588442")
	data[2].SetString("21761276089180230617904476026690048826689721630933485969915548849196498965166")

	h := NewMiMC("mimc")
	h.Write(data[0].Marshal())
	h.Write(data[1].Marshal())
	h.Write(data[2].Marshal())

	r := h.Sum(nil)

	var b big.Int
	b.SetBytes(r)
	fmt.Printf("%s\n", b.String())

outputs 7018430971635270450834876734052312213264841464458602476370836768879919501996

, as does this implementation ->
on python with DEFAULT_EXPONENT = 5 and DEFAULT_SEED = "mimc":

In [3]: mimc_hash([
   ...: 10909369219534740878285360918369814291778422174980871969149168794639722256599,
   ...: 3811523387212735178398974960485340561880938762308498768570292593755555588442,
   ...: 21761276089180230617904476026690048826689721630933485969915548849196498965166])
Out[3]: 7018430971635270450834876734052312213264841464458602476370836768879919501996

[gbotrel]

@ThomasPiellard ?