[issue1623] Implement PEP-3141 for Decimal

[Mark Dickinson]

Mark Dickinson added the comment:

Some more comments:

(1) I don't think I understand the mechanism by which __lt__ and __le__ are supposed to 
be called.  For example, by adding a few print statements it seems that the comparison

Decimal(2) > 3

doesn't call __lt__ at any stage.  Is this what's expected, or is this unintended?

(2) Also on the subject of __le__ and __lt__:  if Decimal is going to have rich 
comparisons (and I guess it is, one way or another :) ) then we might as well get them 
right for NaNs: IEEE-754(r) says that any comparison involving NaN should return False, 
with the exception of != which should always return True.  With 3-way comparison it 
wasn't possible to get this right.  This would fix Issue 1514428.  (Further discussion of 
this should probably go there---I don't think it should hold up applying this patch...)

(3) I'm getting a serious performance hit from this patch:  a complete run of 
test_decimal.py costs me 22.439 seconds (best of 3) with the patch, and 11.604 seconds 
without.  I'm not sure where this is coming from---perhaps it's unavoidable.

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