Genta Kamitani

Results 3 issues of Genta Kamitani

There is no way to set `ValidationHelper.nowFunc` although jwt-go provides an internal way to override `time.Now` by setting `nowFunc`. So I added a new ParserOption `WithTimeFunc` to utilize the feature...

I found a bug with `remove_constant` and GC. I wll later give an example that potentially causes this bug.

## Goal `**` is supported by https://github.com/bugsnag/bugsnag-go/pull/25, but comments are not updated. So I updated the comment.

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