WIP: allow libcst_transform to work piecewise instead of all-or-nothing

[devdanzin]

This PR is meant to help explore a solution for #12. To simplify testing, it restricts running of passes to only replace_bodies_with_ellipsis in initial_cut. It attempts to make libcst_transform work with partial replacements instead of in a all-or-nothing fashion.

The code it's able to reduce is some variation of:

import asyncio
import _lsprof

if True:
    a = 1
    b = 2
    c = 3

if True:
    a = 11
    b = 12
    c = 13

if True:
    obj = _lsprof.Profiler()
    obj.enable()
    obj._pystart_callback(lambda: 0, 0)
    obj = None  
    loop = asyncio.get_event_loop()

if True:
    a = 21
    b = 22
    c = 23

if True:
    a = 31
    b = 32
    c = 33

if True:
    a = 41
    b = 42
    c = 43

if True:
    a = 51
    b = 52
    c = 53

if True:
    a = 61
    b = 62
    c = 63

if True:
    a = 71
    b = 72
    c = 73

It can be run against the following Python version which, on Linux, aborts with the code above:

Python 3.12.8+ (tags/v3.12.8-22-gb0615a8a9aa:b0615a8a9aa, Mar  6 2025, 21:52:08) [GCC 11.4.0]

For that, a interestingness test looks like:

#!/bin/bash
/home/danzin/projects/cpython/python ./short_source_lsprof.py
if ! test "$?" = "134"; then
    exit 1
else
    exit 0
fi

But it might be easier to just substitute the is_interesting function for the equivalent of lambda x: b"Profiler" in x.

The code as-is is known to be broken, since it's not able to generate each substitution and record a reduction from it. But it shows the idea might work if implemented correctly, something along the lines of:

• Count the nodes in the test case;
• Transform the module replacing a single/some nodes and check interestingness;
• Each successful (i.e., interesting) replacement should update the test case, leading to incremental reductions;
• Repeat.

As-is, the code is able to reduce the example above to:

import asyncio
import _lsprof

if True:
    a = 1
    b = 2
    c = 3

if True:
    a = 11
    b = 12
    c = 13

if True:
    obj = _lsprof.Profiler()
    obj.enable()
    obj._pystart_callback(lambda: 0, 0)
    obj = None  
    loop = asyncio.get_event_loop()

if True:
    ...

if True:
    ...

if True:
    ...

if True:
    ...

if True:
    ...

if True:
    ...