add support to cast int64 slices

[Pantani]

What does this MR does?

Create ToInt64SliceE and ToInt64Slice method to support int64 slice casting;

How to test?

• Run the unit tests:

go test -v  -run TestToInt64SliceE .

• Test code:

input := []interface{}{1.2, 3.2}
v = ToInt64Slice(input)
fmt.Println(v)

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[Pantani]

@bep, can you help me to review, please?