Set pypa option for not handling future import errors

Signed-off-by: Vinzenz Feenstra <evilissimo@gmail.com>
Astprint improvements

Signed-off-by: Vinzenz Feenstra <evilissimo@gmail.com>
Fix assert in very verbose mode
Fix slicing issues
In addition:
 * unify slicing
 * performance improvements, especially for string slicing
fix memory leak
Add repr ics and refactor things slightly
Merge pull request #219 from undingen/mem_leak

fix memory leak
Merge pull request #214 from vinzenz/astprint-improvements

Astprint improvements
Merge pull request #215 from vinzenz/pypa-future-handling

Set pypa option for not handling future import errors
Merge pull request #217 from undingen/llvm_assert

Fix assert in very verbose mode
Merge pull request #218 from undingen/fix_slice

Fix slicing issues
List slice-self-assignment is currently buggy

Not sure how hard a requirement it is to match CPython's behavior,
but just assert out for now rather than doing something different.

Also add an assert to copySlice, which could have similar issues.
Minor: call this _doFree helper from this other path
Try using obj_idx to index into the free bitfield instead of atom_idx