Symbol: split into Symbol & Dummy for efficiency
Symbol: split into Symbol & Dummy for efficiency

Let's consider a situation when we create a lot of dummies.

When hash(x) is called, we always see x.is_dummy is true or false.

accessing .is_dummy is expensive (it goes through assumptions code), and always
storing .is_dummy as instance attribute adds it's own overhead.

Why not to split Symbol into Symbol & Dummy and implement _hashable_content
differently, and have .is_dummy as class attribute?

This positively affects performance:

    time ./examples/fem_test.py

old:    6.03s
new:    5.83s

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