Add `mapExists` function

[yukikurage]

Description of the change

fix #16

And the documentation was a bit wrong, so I fixed it.

nats :: Stream Number
nats = mkExists $ StreamF 0 (\n -> Tuple (n + 1) n)

↓

nats :: Stream Int
nats = mkExists $ StreamF 0 (\n -> Tuple (n + 1) n)

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Checklist:

• [ ] Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• [ ] Linked any existing issues or proposals that this pull request should close
• [ ] Updated or added relevant documentation
• [ ] Added a test for the contribution (if applicable)