Allow mock_callable() to work with async functions

[fornellas]

Add support for mock_callable() to wok on async methods:

import testslide, asyncio, inspect

class Test(object):
  @classmethod
  async def m(self):
    return 'original'

class TestAsync(testslide.TestCase):
  def test_async(self):
    self.assertEqual(
      asyncio.run(Test.m()),
      'original'
    )

    self.mock_callable(Test, 'm')\
      .to_return_value('mock')
    
    self.assertEqual(
      asyncio.run(Test.m()),
      'mock'
    )

In principle, this should work for all cases (functions, instance methods, class methods etc), but we need to add test coverage.

Things to consider:

• with_implementation and with_wrapper should ideally also accept async methods as arguments.
• to_call_original might be broken for async, we need a test there.