[#20266] Fix bugs decoding integers and fixed bytes in indexed event fields

[shoenseiwaso]

Resolves #20266

This PR fixes bugs in parseTopics() and parseTopicsIntoMap() which incorrectly decoded negative integer values and fixed bytes values.

Added tests, now passing, which fail on master:

--- FAIL: TestParseTopics (0.00s)
    --- PASS: TestParseTopics/support_fixed_byte_types,_right_padded_to_32_bytes (0.00s)
    --- FAIL: TestParseTopics/int8_with_negative_value (0.00s)
        topics_test.go:142: parseTopics() = &{115792089237316195423570985008687907853269984665640564039457584007913129639934}, want &{-1}
    --- FAIL: TestParseTopics/int256_with_negative_value (0.00s)
        topics_test.go:142: parseTopics() = &{115792089237316195423570985008687907853269984665640564039457584007913129639934}, want &{-1}
--- FAIL: TestParseTopicsIntoMap (0.00s)
    --- FAIL: TestParseTopicsIntoMap/support_fixed_byte_types,_right_padded_to_32_bytes (0.00s)
        topics_test.go:181: parseTopicsIntoMap() = map[staticBytes:[1 2 3 4 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]], want map[staticBytes:[1 2 3 4 5]]
    --- FAIL: TestParseTopicsIntoMap/int8_with_negative_value (0.00s)
        topics_test.go:181: parseTopicsIntoMap() = map[int8Value:115792089237316195423570985008687907853269984665640564039457584007913129639935], want map[int8Value:-1]
    --- FAIL: TestParseTopicsIntoMap/int256_with_negative_value (0.00s)
        topics_test.go:181: parseTopicsIntoMap() = map[int256Value:115792089237316195423570985008687907853269984665640564039457584007913129639935], want map[int256Value:-1]

This PR promotes a few functions and variables to public, all inaccounts/abi/unpack.go, to help reduce duplication in decoding fields: MaxUint256, MaxInt256, ReadInteger() and ReadFixedBytes().

Future enhancement, not addressed in this PR, could further reduce the duplication of functionality between parseTopics(), parseTopicsIntoMap() and toGoType().

[holiman]

LGTM

[holiman]

Thanks! Please take a look at and fix the lint failures on travis

[shoenseiwaso]

Thanks for the quick turnaround! Linter issues fixed and linting and tests passed in CI.

[gballet]

LGTM once updated