[HACKERS] WIP Patch: Precalculate stable functions, infrastructure v1

Hello everyone again!

This is the continuation of my previous patch on the same topic; here 
there are changes made thanks to Tom Lane comments (see thread here 
[1]). To not send big patch I have split it (that's why version starts 
with the first again) and here I send infrastructure patch which 
includes:
- creation of CachedExpr node
- usual node functions for it
- mutator to replace nonovolatile functions' and operators' expressions 
by appropriate cached expressions.

Any suggestions are welcome!

[1] 

https://www.postgresql.org/message-id/flat/98c77534fa51aa4bf84a5b39931c42ea%40postgrespro.ru#98c77534fa51aa4bf84a5b39931c42ea@postgrespro.ru

-- 
Marina Polyakova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
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