polys: rewrite _swap function in modulargcd
polys: rewrite _swap function in modulargcd

No more conversions to dense representation are used for this function.
saurabhjn76/sympy,MridulS/sympy,Mitchkoens/sympy,mcdaniel67/sympy,farhaanbukhsh/sympy,iamutkarshtiwari/sympy,shikil/sympy,mcdaniel67/sympy,shikil/sympy,vipulroxx/sympy,lindsayad/sympy,hrashk/sympy,drufat/sympy,souravsingh/sympy,oliverlee/sympy,saurabhjn76/sympy,postvakje/sympy,atreyv/sympy,lidavidm/sympy,madan96/sympy,AunShiLord/sympy,liangjiaxing/sympy,skirpichev/omg,hrashk/sympy,Gadal/sympy,skidzo/sympy,chaffra/sympy,cccfran/sympy,atreyv/sympy,shipci/sympy,MridulS/sympy,Sumith1896/sympy,Designist/sympy,lidavidm/sympy,atsao72/sympy,cccfran/sympy,VaibhavAgarwalVA/sympy,abhiii5459/sympy,emon10005/sympy,cswiercz/sympy,kaichogami/sympy,kaichogami/sympy,Davidjohnwilson/sympy,wanglongqi/sympy,skidzo/sympy,kmacinnis/sympy,meghana1995/sympy,postvakje/sympy,iamutkarshtiwari/sympy,meghana1995/sympy,Sumith1896/sympy,MechCoder/sympy,bukzor/sympy,ahhda/sympy,yukoba/sympy,pandeyadarsh/sympy,dqnykamp/sympy,hrashk/sympy,ahhda/sympy,Vishluck/sympy,sahilshekhawat/sympy,cccfran/sympy,Arafatk/sympy,kaushik94/sympy,sahmed95/sympy,Sumith1896/sympy,kaushik94/sympy,atreyv/sympy,lindsayad/sympy,emon10005/sympy,wyom/sympy,MridulS/sympy,vipulroxx/sympy,pandeyadarsh/sympy,postvakje/sympy,hargup/sympy,Arafatk/sympy,madan96/sympy,farhaanbukhsh/sympy,kaushik94/sympy,sunny94/temp,abhiii5459/sympy,kevalds51/sympy,souravsingh/sympy,toolforger/sympy,kumarkrishna/sympy,MechCoder/sympy,beni55/sympy,kumarkrishna/sympy,grevutiu-gabriel/sympy,AunShiLord/sympy,Titan-C/sympy,pandeyadarsh/sympy,moble/sympy,jbbskinny/sympy,jerli/sympy,sunny94/temp,hargup/sympy,ChristinaZografou/sympy,aktech/sympy,AunShiLord/sympy,diofant/diofant,sampadsaha5/sympy,AkademieOlympia/sympy,Designist/sympy,hargup/sympy,emon10005/sympy,garvitr/sympy,Shaswat27/sympy,wanglongqi/sympy,maniteja123/sympy,aktech/sympy,Vishluck/sympy,Arafatk/sympy,meghana1995/sympy,kumarkrishna/sympy,shipci/sympy,Davidjohnwilson/sympy,sampadsaha5/sympy,debugger22/sympy,jerli/sympy,madan96/sympy,cswiercz/sympy,toolforger/sympy,abhiii5459/sympy,jaimahajan1997/sympy,sahmed95/sympy,atsao72/sympy,liangjiaxing/sympy,ga7g08/sympy,liangjiaxing/sympy,lidavidm/sympy,grevutiu-gabriel/sympy,saurabhjn76/sympy,jamesblunt/sympy,abloomston/sympy,wanglongqi/sympy,Titan-C/sympy,Curious72/sympy,moble/sympy,chaffra/sympy,ahhda/sympy,oliverlee/sympy,kevalds51/sympy,kmacinnis/sympy,asm666/sympy,chaffra/sympy,pbrady/sympy,skidzo/sympy,garvitr/sympy,yashsharan/sympy,maniteja123/sympy,dqnykamp/sympy,sunny94/temp,AkademieOlympia/sympy,beni55/sympy,Mitchkoens/sympy,mafiya69/sympy,beni55/sympy,AkademieOlympia/sympy,iamutkarshtiwari/sympy,jaimahajan1997/sympy,Shaswat27/sympy,debugger22/sympy,Designist/sympy,souravsingh/sympy,mafiya69/sympy,vipulroxx/sympy,Gadal/sympy,lindsayad/sympy,ChristinaZografou/sympy,VaibhavAgarwalVA/sympy,bukzor/sympy,sahilshekhawat/sympy,Gadal/sympy,Shaswat27/sympy,ga7g08/sympy,aktech/sympy,farhaanbukhsh/sympy,yashsharan/sympy,jaimahajan1997/sympy,asm666/sympy,Curious72/sympy,jbbskinny/sympy,mcdaniel67/sympy,shipci/sympy,shikil/sympy,jbbskinny/sympy,sampadsaha5/sympy,MechCoder/sympy,jerli/sympy,kevalds51/sympy,grevutiu-gabriel/sympy,mafiya69/sympy,toolforger/sympy,cswiercz/sympy,wyom/sympy,oliverlee/sympy,maniteja123/sympy,Mitchkoens/sympy,dqnykamp/sympy,moble/sympy,rahuldan/sympy,drufat/sympy,ga7g08/sympy,Titan-C/sympy,pbrady/sympy,kmacinnis/sympy,Davidjohnwilson/sympy,garvitr/sympy,drufat/sympy,ChristinaZografou/sympy,Vishluck/sympy,yukoba/sympy,abloomston/sympy,yukoba/sympy,VaibhavAgarwalVA/sympy,sahilshekhawat/sympy,atsao72/sympy,rahuldan/sympy,Curious72/sympy,jamesblunt/sympy,wyom/sympy,kaichogami/sympy,abloomston/sympy,sahmed95/sympy,yashsharan/sympy,asm666/sympy,debugger22/sympy,rahuldan/sympy,pbrady/sympy,jamesblunt/sympy,bukzor/sympy