bpo-17611. Move unwinding of stack for "pseudo exceptions" from interpreter to compiler.

[markshannon]

See bpo-17611 for detailed discussion.
This PR:

• Handles stack unwinding in the compiler rather than the interpreter, getting rid of "pseudo exceptions".
• Deletes a number of overly complicated bytecodes, either eliminating them entirely or replacing them with simpler alternatives.
• Removes one field from PyTryBlock reducing the size of a frame by 80 bytes.

This PR is not yet ready for merging as it does not include a misc.news entry, updates to Docs/library/dis.rst and the dis module tests fail.
If it is agreed that this is to be merged, then I will fill in the missing parts.

There are also a couple of further improvements that I would like to make, but have intentionally omitted. I intend to add them in separate PRs:

• Replace LOAD_ADDR with LOAD_CONST in the peephole optimiser.
• Fix stack size computation.

https://bugs.python.org/issue17611

[pitrou]

It looks like the build core dumped on Travis-CI. There's also a build issue on AppVeyor (perhaps related to ..\Objects\frameobject.c(10): fatal error C1083: Cannot open include file: 'Python/wordcode_helpers.h': No such file or directory [C:\projects\cpython\PCbuild\pythoncore.vcxproj]).

[nascheme]

The build error can be fixed by moving wordcode_helpers.h into Include. Then change "Python/wordcode_helpers.h" in frameobject.c to "wordcode_helpers.h". The current code doesn't compile if your build directory is not in the top-level source directory.

The following code causes a weird error in traceback.py:

def f1():                                                                   
    try:                                                                    
        raise RuntimeError                                                  
    finally:                                                                
        return 1                                                            
f1()                                                                        
                                                                            
def f2():                                                                   
    try:                                                                    
        raise Exception                                                     
    except:                                                                 
        try:                                                                
            x = 1                                                           
        finally:                                                            
            return 1                                                        
f2()                                                                        
                                                                            
try:                                                                        
    assert 0, 'assert #2'                                                   
except:                                                                     
    import traceback, sys                                                   
    print(sys.exc_info())                                                   
    traceback.print_exc()   

[markshannon]

Closing in favour of #5006 and #5112