Speed up cache decorator
Speed up cache decorator

This makes all cache retrievals faster, eg:

before
In [1]: %timeit Pow(y, 2)
100000 loops, best of 3: 9.8 us per loop

after
In [1]: %timeit Pow(y, 2)
100000 loops, best of 3: 7.5 us per loop

shipci/sympy,grevutiu-gabriel/sympy,yukoba/sympy,kmacinnis/sympy,liangjiaxing/sympy,kevalds51/sympy,dqnykamp/sympy,liangjiaxing/sympy,postvakje/sympy,MridulS/sympy,kaichogami/sympy,oliverlee/sympy,VaibhavAgarwalVA/sympy,wyom/sympy,aktech/sympy,saurabhjn76/sympy,jerli/sympy,moble/sympy,rahuldan/sympy,skidzo/sympy,shipci/sympy,AkademieOlympia/sympy,debugger22/sympy,Davidjohnwilson/sympy,chaffra/sympy,toolforger/sympy,MechCoder/sympy,kaushik94/sympy,postvakje/sympy,sampadsaha5/sympy,moble/sympy,bukzor/sympy,aktech/sympy,mcdaniel67/sympy,Mitchkoens/sympy,Davidjohnwilson/sympy,ga7g08/sympy,souravsingh/sympy,yukoba/sympy,grevutiu-gabriel/sympy,hargup/sympy,Gadal/sympy,sunny94/temp,jamesblunt/sympy,hrashk/sympy,skidzo/sympy,abloomston/sympy,Mitchkoens/sympy,atreyv/sympy,farhaanbukhsh/sympy,Designist/sympy,Designist/sympy,diofant/diofant,Arafatk/sympy,saurabhjn76/sympy,Shaswat27/sympy,shipci/sympy,sahilshekhawat/sympy,ahhda/sympy,maniteja123/sympy,yukoba/sympy,bukzor/sympy,shikil/sympy,sampadsaha5/sympy,ga7g08/sympy,rahuldan/sympy,wyom/sympy,cccfran/sympy,cswiercz/sympy,Curious72/sympy,vipulroxx/sympy,lindsayad/sympy,jaimahajan1997/sympy,maniteja123/sympy,Sumith1896/sympy,VaibhavAgarwalVA/sympy,jbbskinny/sympy,hrashk/sympy,meghana1995/sympy,wanglongqi/sympy,AkademieOlympia/sympy,drufat/sympy,ahhda/sympy,atsao72/sympy,lidavidm/sympy,beni55/sympy,meghana1995/sympy,wanglongqi/sympy,mafiya69/sympy,abhiii5459/sympy,sahilshekhawat/sympy,abhiii5459/sympy,hrashk/sympy,pbrady/sympy,lidavidm/sympy,souravsingh/sympy,grevutiu-gabriel/sympy,wyom/sympy,kaushik94/sympy,garvitr/sympy,meghana1995/sympy,souravsingh/sympy,srjoglekar246/sympy,maniteja123/sympy,Gadal/sympy,jamesblunt/sympy,lindsayad/sympy,ChristinaZografou/sympy,shikil/sympy,Titan-C/sympy,bukzor/sympy,pandeyadarsh/sympy,atsao72/sympy,amitjamadagni/sympy,Mitchkoens/sympy,sampadsaha5/sympy,MechCoder/sympy,toolforger/sympy,asm666/sympy,chaffra/sympy,mafiya69/sympy,yashsharan/sympy,Sumith1896/sympy,dqnykamp/sympy,madan96/sympy,emon10005/sympy,Titan-C/sympy,MechCoder/sympy,drufat/sympy,jbbskinny/sympy,hargup/sympy,debugger22/sympy,farhaanbukhsh/sympy,ChristinaZografou/sympy,cccfran/sympy,farhaanbukhsh/sympy,debugger22/sympy,emon10005/sympy,Curious72/sympy,MridulS/sympy,Vishluck/sympy,Gadal/sympy,moble/sympy,jaimahajan1997/sympy,rahuldan/sympy,pbrady/sympy,iamutkarshtiwari/sympy,Vishluck/sympy,skidzo/sympy,wanglongqi/sympy,cswiercz/sympy,sahilshekhawat/sympy,postvakje/sympy,sahmed95/sympy,atreyv/sympy,Arafatk/sympy,AunShiLord/sympy,kevalds51/sympy,Designist/sympy,madan96/sympy,Vishluck/sympy,cccfran/sympy,atsao72/sympy,asm666/sympy,skirpichev/omg,kmacinnis/sympy,abloomston/sympy,beni55/sympy,mcdaniel67/sympy,saurabhjn76/sympy,amitjamadagni/sympy,yashsharan/sympy,Titan-C/sympy,pbrady/sympy,vipulroxx/sympy,dqnykamp/sympy,oliverlee/sympy,sunny94/temp,abhiii5459/sympy,pandeyadarsh/sympy,kumarkrishna/sympy,beni55/sympy,kaichogami/sympy,madan96/sympy,hargup/sympy,AunShiLord/sympy,sahmed95/sympy,atreyv/sympy,ChristinaZografou/sympy,lindsayad/sympy,mcdaniel67/sympy,aktech/sympy,toolforger/sympy,Curious72/sympy,iamutkarshtiwari/sympy,oliverlee/sympy,AunShiLord/sympy,ga7g08/sympy,VaibhavAgarwalVA/sympy,vipulroxx/sympy,lidavidm/sympy,AkademieOlympia/sympy,iamutkarshtiwari/sympy,jerli/sympy,jbbskinny/sympy,sunny94/temp,Arafatk/sympy,jaimahajan1997/sympy,cswiercz/sympy,Shaswat27/sympy,kumarkrishna/sympy,jerli/sympy,mafiya69/sympy,kevalds51/sympy,jamesblunt/sympy,abloomston/sympy,garvitr/sympy,emon10005/sympy,flacjacket/sympy,kaichogami/sympy,Shaswat27/sympy,kaushik94/sympy,asm666/sympy,kmacinnis/sympy,pandeyadarsh/sympy,shikil/sympy,ahhda/sympy,kumarkrishna/sympy,Sumith1896/sympy,MridulS/sympy,yashsharan/sympy,garvitr/sympy,chaffra/sympy,liangjiaxing/sympy,drufat/sympy,Davidjohnwilson/sympy,sahmed95/sympy