Update Marshal method for the fields to accept Go native types

[alovak]

We want to support Go's native types such as string, int, etc. when we set data to the field using Marshal method

So, the idea is to implement the following:

f := field.NewString(someSpec)

err := f.Marshal("hello")
f.Value() // "hello"

err := f.Marshal(5)
f.Value() // "5"

str := "hello"
err := f.Marshal(&str)
f.Value() // "hello"

i := 5
err := f.Marshal(&i)
f.Value() // "5"

We want to be able to use:

• int, *int, string, *string for field.Numeric, field.String
• []byte, string for field.Binary, field.Hex (string should be hex and if we fail to decode it, return error)

Reference implementation is in this PR: https://github.com/moov-io/iso8583/pull/273/files

[alovak]

closed by #289