Overflow protection and fuzz tests for gas meter & std dev filter

[hacheigriega]

• Added overflow protections in tally and pubkey modules.
• Fuzz testing of tally gas meter and standard deviation particularly involving large numbers
• Fix the number parsing of standard deviation filter to follow the pattern of ojg JSON parser, which parses numbers in the range of -9223372036854775799 and 9223372036854775799 as int64 and the numbers out of that range as strings (encoding/json.Number).