Add lemma pmf_measurable

[Yosuke-Ito-345]

Motivation for this change

Prove that the function pmf is measurable.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md
• added corresponding documentation in the headers

Reference: How to document

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[Yosuke-Ito-345]

@affeldt-aist @t6s
I kindly ask for your review of this pull request.
Please note that I moved the definition of pmf outside of the Section discrete_distribution, because the random variable here need not be discrete and I'm planning to use the lemma pmf_measurable for a general random variable.