4318: prototyp.BigInt: fix int64 parsing overflow

[david-littlefarmer]

Ticket: https://github.com/0xsequence/issue-tracker/issues/4318

Previously, parsing a string into BigInt would silently return 0 on overflow of int64 without any warning. This fix ensures that numbers larger than int64 are correctly parsed instead of returning 0, because of overflow.

Also added handling of negative numbers and tests to cover edge cases related to int64 parsing.

[VojtechVitek]

Looks like you fixed NewBigIntFromString() for all big numbers. What a great bugfix, David. Excellent work!

LGTM

[VojtechVitek]

TL;DR: This PR fixes the following bug:

func TestPrototyp(t *testing.T) {
	a := prototyp.NewBigIntFromString("029a2241af62c000c8", 16)
	b := prototyp.NewBigIntFromString("0x029a2241af62c000c8", 16)
	t.Logf("a: %s", a.String())
	t.Logf("b: %s", b.String())
}

=== RUN   TestPrototyp
    prototyp_test.go:12: a: 48000000000000000200
    prototyp_test.go:13: b: 0