demandimport: support importing builtins for Python 3 __builtin__ was renamed to builtins in Python 3. If importing __builtin__ fails, fall back to importing builtins in order to support Python 3.

demandimport: alias __builtin__ as builtins Python 3 renamed the __builtin__ module to builtins. In preparation for supporting Python 3, alias the imported module as "builtins."

bufferedinputpipe: remove N^2 computation of buffer length (issue4735) The assumption that dynamically computing the length of the buffer was N^2, but negligible because fast was False. So we drop the dynamic computation and manually keep track of the buffer length.