assumptions: cache 'unknown' assumptions as well
assumptions: cache 'unknown' assumptions as well

Unknown assumptions usually take the highest time to evaluate, and we already
cache known assumptions, so there is no reason why not to cache unknowns.

This speed up things a bit:

x = Symbol('x')

        timeit          py.test
        x.is_positive   sympy/core/

old:    115 µs          13.8s
new:     21 µs          12.8s

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