Jeremy, please fix this correctly after the alpha.

tracing/line number table in except blocks.

Reflow long lines introduced by col_offset changes.  Update test_ast
to handle new fields in excepthandler.

As note in Python.asdl says, we might want to rethink how attributes
are handled.  Perhaps they should be the same as other fields, with
the primary difference being how they are defined for all types within
a sum.

Also fix asdl_c so that constructors with int fields don't fail when
passed a zero value.

 - IMPORT_NAME takes an extra argument from the stack: the relativeness of
   the import. Only passed to __import__ when it's not -1.

 - __import__() takes an optional 5th argument for the same thing; it
   __defaults to -1 (old semantics: try relative, then absolute)

 - 'from . import name' imports name (be it module or regular attribute)
   from the current module's *package*. Likewise, 'from .module import name'
   will import name from a sibling to the current module.

 - Importing from outside a package is not allowed; 'from . import sys' in a
   toplevel module will not work, nor will 'from .. import sys' in a
   (single-level) package.

 - 'from __future__ import absolute_import' will turn on the new semantics
   for import and from-import: imports will be absolute, except for
   from-import with dots.

Includes tests for regular imports and importhooks, parser changes and a
NEWS item, but no compiler-package changes or documentation changes.

be complete when when checking types.

Yield cannot be tested outside a function, so add a comment to that effect.

a tree of Python objects. Expose this through compile().