[Pyparsing] packrat bug + various

[Paolo L. <p....@hy...>]

Hello Paul and everyone,
	I'm trying to upgrade my application from 1.3.3 to 1.4.3,
mainly for the PackRat feature since I expect huge performance gain from 
it in my app. I'm finding some problems/bugs though...

### PackRat bug

reuse of ParseResults strategy conflicts with PackRat ParseResult 
caching. For an example:
-----------------------
from pyparsing import *

A = Literal('A')
B = Literal('B')
C = Literal('C')

e = A ^ ( A + B + C )

print e.parseString("AB")
ParserElement.enablePackrat()
print e.parseString("AB")
-------------------------


The solution could be dropping the reuse strategy (drop __iadd__
and __new__ from ParseResults). I think that this not only would solve
the bug but would also make the code easier to understand/maintain with 
a little sacrifice in performance (has the performance advantage ever 
been measured?). I know this is a drastic solution and I want to hear 
from you Paul since I'd like to keep in synch with your official releases...

### skipWhitespace bug

I attach a small patch related to whitespaces skipping...

### PackRat threading enhancement

Current packrat implementation seems to be thread safe but not thread 
friendly: a new thread, via resetCache(), forces a global cache flush
also when other threads are running... I suggest making resetCache 
invocation explicit or make specific per thread cache... what do you think?

### Performance and simple code

I see that in some points of the code (i.e. ParserElement._parse)
some performance optimization is achieved by making the code a little 
bit more complex... IMVHO performance optimization should be obtained 
implementing the critical code in C (many other python projects follow 
the same rule: in python reference clean code implementation, in C 
functionally equivalent fast code.
I volunteer for C implementation if we agree on the approach...
main targets could be ParseResults and ParserElement._parse...

Thank you very much for the fantastic work

	Ciao
	Paolo