Add Python package module for Python+Chapel interop

[jabraham17]

Adds a new package module, Python, which supports calling Python code from Chapel.

For example, the following Chapel progam use the python interface to BeautifulSoup to parse HTML

import Python;
use URL;
use List;

proc main() {

  const url = 'https://chapel-lang.org/docs/main/language/spec/interoperability.html';
  var htmlReader = openUrlReader(url);
  var html = htmlReader.readAll(string);

  var interp = new Python.Interpreter();
  var mod = new Python.Module(interp, "bs4");

  var cls = new Python.Class(mod, "BeautifulSoup");
  var soup = cls(html, 'html.parser');

  var res: list(owned Python.ClassObject?);
  res = soup.callMethod(res.type, "find_all", "h3");
  for c in res {
    writeln(c!.getAttr(string, "text"));
  }
}

As an another example, this simple program compiles an runs a Python lambda from Chapel code

import Python;

config const n = 10;
config const func = "lambda x,: x + 1 if x % 2 != 0 else x";

proc apply(interp: borrowed, type t, arr, l) {
  var lambdaFunc = new Python.Function(interp, l);
  var res: [arr.domain] t;
  for i in arr.domain {
    res(i) = lambdaFunc(t, arr(i));
  }
  return res;
}
proc main() {
  var interp = new Python.Interpreter();

  var data: [1..#n] int = 1..#n;
  writeln(" data: ", data);

  var res = apply(interp, int, data, func);
  writeln("res: ", res);
}

Future work:

• Convert classes to a hierarchy under Value so that more implementation can be shared and the Chapel interface can better imitate python
  • Add special support for common python types as sub-classes of value, to avoid round tripping values through chapel in many cases
  • Support common operators like add. These can be called today as dunder methods (.call("__add__", ...)), but it would be nice to support native operators like +
• Add the ability to compile arbitrary chapel strings, beyond just lambdas
• Setup python to use Chapel stdout/stderr
• Add custom adapters for Chapel arrays to allow python functions to operate on Chapel arrays, without copying
• Support python context managers as Chapel context managers
• Fully support Python sets, Python dictionaries, Chapel sets, Chapel maps, Chapel associative arrays, and Chapel associative domains
• Make sure all python objects are properly reference counted
• Resolve TODO with LLVM IR verification