Add helper to go from []T to []*T for string, int64, float64, bool

[sobel-stripe]

I ran into a common mistake in golang when writing my own code to go from a []string to []*string for use in the PaymentMethodTypes field.

This adds a helper function to do it correctly. If we want to merge this PR, I also think that we should update the examples in the documentation to use it.

[remi-stripe]

r? @brandur-stripe Do you think we should add this? If so we would need to also update all tests to use this instead of the current approach.

[brandur-stripe]

Yeah, this makes sense to me — the AWS SDK has it already, and it'll make a lot of our test code cleaner as it becomes possible to leave out all the individual elements' own stripe.String.

@sobel-stripe Thanks! Would you mind updating this so that there's one for all the types that we support? (e.g. Int64Slice) It shouldn't just be a string-specific construct.

[sobel-stripe]

@brandur-stripe done, and updated the test call sites [could only find string] as well

[brandur-stripe]

Thanks @sobel-stripe (and wow, thanks a lot for updating all those other tests).

Sorry to be a pain, but one more small-ish ask: would you mind putting tests in stripe_test.go for all the different new functions for non-string types as well? I know the type system pretty much guarantees that we're good here, but just seems good for general hygiene.

ptal @sobel-stripe

[sobel-stripe]

@brandur-stripe no problem — added the tests

[brandur-stripe]

Thanks @sobel-stripe! LGTM.

[brandur-stripe]

Released as 60.6.0.