Move some type-related stuff into typeobject.cpp or descrobject.cpp

Separated out from some more involved changes that are
required to get the 'pwd' module working.

Haven't added structseq.c to the build quite yet

Remove some definitions that should no longer be in there
(structs for which we have Pyston-internal definitions).

Instead of doing "typedef void PyIntObject", do
"struct _PyIntObject; typedef struct _PyIntObject PyIntObject;"
which prevents spurious things like type conversions or
sizeof(void)'s.

I'm not sure that we need to do this at the expression-level --
I think we just have to do it at the statement level.

ie 'from a import b' now attempts an 'import a.b' if b does not already exist.
This also works for imports caused by__all__.

Previously we were calling it just 'b'.

ScopeInfo involves checking the whole function subtree to resolve
scoping references.  SourceInfo has information about the specific
function.  is_generator used to be in ScopeInfo, but accessing it
would require doing the full subtree analysis, which can be
unnecessary.

This lets us avoid analyzing function subtrees that are never entered.

With some changes
- renamed AST_Exec::expr -> body (to match CPython)
- removed references to AST_SetComp

add two passes which remove unnecessary boxing

* the first one removes boxInt, boxFloat and boxBool calls where the argument is coming from a corresponding unbox call
* the second pass removes duplicate boxing calls inside the same BB

together they improve the performance by about 10%

If we had to guard on the type of an object but the variable
ended up being undefined, we treated that as a guard failure
(not sure why).

Fixing that exposed another issue: if we guard that an object
is an int, we will try to unbox it if necessary.  If the variable
wasn't defined, then we will try to unbox some garbage memory.
Use the new handlePotentiallyUndefined to deal with that.

Before, for compiled functions we used dwarf line number information.

Now it's switched to using the new frame introspection support, and
we pass through the AST_stmt* as one of the arguments.  This gets us
the line number and also the definition of execution point.

and exc_info -> unw_info

(This makes more sense with the next change, but I wanted
to separate this large renaming from the functional change.)

I wonder if other people have run into this.

Previously things were hardcoded to only allow a single entry point to
the interpreter (since that is how the unwinder determines if the frame
is an interpreter frame).

Instead of having astInterpretFunction be the interpreter frame, make
execute() the frame.  Have to make it a static class function so we can
take its address.

Can still switch back to the default regalloc using the -b flag,
but it looks like the basic register allocator cuts the LLVM JIT
time significantly, and improves every one of our benchmarks.

With the recent package import support, we broke importing optparse.
This is because we now can import "encodings" (a package), which imports
codecs, which raises a SystemError when trying to import _codecs which
we don't support yet.

This was getting ignored before since optparse wrapped this process
(which happens via "import gettext") in a try-except and catches the
ImportError.