Unpythonic

- Add batteries for functools: `unpythonic.fun`
- Add `cons` and friends: `unpythonic.llist` (not loaded by default; depends on `unpythonic.tco`)
- Bugfix: `rc.py` was missing from the distribution, breaking TCO

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Bugfix:

- Catch `UnknownArity` in all modules that use `unpythonic.arity.arity_includes()`.

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New and improved sequencing constructs.

- Rename the v0.5.1 `do` --> `pipe`
- Add `piped`, `lazy_piped` for shell-like pipe syntax
- Add a new `do`: an improved `begin` that can name intermediate results

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- Catch more errors in client code:
- Validate arity of callable value in `letrec` (both versions)
- Validate callability and arity of body in all `let` constructs
- In `env`, require name to be an identifier even when subscripting
- Add flag to `enable_fasttco()` to be able to switch it back off (during the same run of the Python interpreter)
- Internal enhancement: add unit tests for `env`

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- Make the TCO implementation switchable at run time, see `enable_fasttco()`.
- Default to the slower exception-based TCO that has easier syntax.

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- Print a warning for unclaimed TCO jump instances (to help detect bugs in client code)

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