New fee table

[krlosMata]

Describe issue

Current fee table values are all shifted left 79 bits in order to have precision for all feeFactors. This leads that the maximum feeFactorShifted value is 10**31 << 79, with a bit length of 182.

Taking into account that maximum transferred amountFloat could be 0xFFFF, with a bit length of 113 bits, computing amount * feeFactorShifted could overflow snark Field maximum bits length (253) since 182 + 113 = 295 > 253. Error is not triggered since num2bits template does not checks overflows and therefore, it could led to applying an erroneous feeAmount

Solution

• fee table would be changed to not reach an overflow situation
• shifted would be applied to fee table if precision is needed
• sanity check on fee computed: fee computed < 2^{129

Implementation

• generalize fee-table-selector into a mux256
• isolate computing fee into a single circuit compute-fee
• update balance-updater with above circuit abstractions
• update fee table values

[krlosMata]

Duplicated in #15