Fix Clang compilation warnings and error handling.

[eltoder]

• setbit() takes int, not long.
• idx_t (index in bitarray) is long long, which can be bigger than Py_ssize_t. Use these types as appropriate.
• Add error checking and range checking to setlist().

[andre-merzky]

Thank you!

[diamondman]

Finished reviewing it. All looks sane. Good catches.