mp.c, catacomb/__init__.py, pyke/: Fix mixed-mode arithmetic involving `float'.

mp.c, catacomb/__init__.py, pyke/: Fix mixed-mode arithmetic involving `float'.

This is a bit embarrassing.

	>>> import catacomb as C
	>>> x = C.MP(5)
	>>> x == 5.1
	True
	>>> x < 5.1
	False
	>>> r = x/2
	>>> r
	5/2
	>>> r == 2
	Traceback (most recent call last):
	  File "<stdin>", line 1, in <module>
	TypeError: an integer is required
	>>> r == 2.5
	Traceback (most recent call last):
	  File "<stdin>", line 1, in <module>
	TypeError: an integer is required
	>>> r*1.0
	5/2
	>>> r*1.1
	5/2
	>>> r*2.0
	5
	>>> r*2.5
	5

Fix this nonsense.

  * Change the `obvious' arithmetic operators so that they notice that
    one of the operands is a float.  Handle this by converting to a
    Python bignum and letting Python handle the arithmetic.  The result
    is a float, which seems sensible inexact contagion.

  * Introduce a rich-comparison method which also detects a float
    operand and hands off to Python.  Python seems to get this right,
    comparing the float to the bignum in its full precision, so that's a
    win.

  * Also, modify the `IntRat' code to apply inexact contagion in the
    same way.  Comparisons may be imperfect here, but that's
    surprisingly hard to get right.

The new results:

	>>> import catacomb as C
	>>> x = C.MP(5)
	>>> x == 5.1
	False
	>>> x < 5.1
	True
	>>> r = x/2
	>>> r
	5/2
	>>> r == 2
	False
	>>> r == 2.5
	True
	>>> r*1.0
	2.5
	>>> r*1.1
	2.75
	>>> r*2.0
	5.0
	>>> r*2.5
	6.25