Make DescriptorImpl support multiple subscripts
Make DescriptorImpl support multiple subscripts

So far, no descriptor exists that supports more than one sub-script
descriptor. This will change with taproot, so prepare for this by
changing the m_subdescriptor_arg from a unique_ptr to a vector of
unique_ptr's.

bitcoin/bitcoin,domob1812/bitcoin,ajtowns/bitcoin,rnicoll/bitcoin,sipsorcery/bitcoin,fujicoin/fujicoin,jlopp/statoshi,AkioNak/bitcoin,achow101/bitcoin,namecoin/namecoin-core,AkioNak/bitcoin,domob1812/namecore,lateminer/bitcoin,yenliangl/bitcoin,sipsorcery/bitcoin,kallewoof/bitcoin,mm-s/bitcoin,mm-s/bitcoin,qtumproject/qtum,sstone/bitcoin,apoelstra/bitcoin,jamesob/bitcoin,domob1812/namecore,instagibbs/bitcoin,domob1812/namecore,mruddy/bitcoin,particl/particl-core,achow101/bitcoin,instagibbs/bitcoin,particl/particl-core,Xekyo/bitcoin,qtumproject/qtum,rnicoll/bitcoin,namecoin/namecoin-core,lateminer/bitcoin,sstone/bitcoin,domob1812/namecore,pataquets/namecoin-core,ajtowns/bitcoin,GroestlCoin/bitcoin,qtumproject/qtum,mm-s/bitcoin,bitcoinsSG/bitcoin,namecoin/namecore,jlopp/statoshi,AkioNak/bitcoin,achow101/bitcoin,GroestlCoin/bitcoin,prusnak/bitcoin,jlopp/statoshi,qtumproject/qtum,rnicoll/bitcoin,rnicoll/bitcoin,namecoin/namecoin-core,dscotese/bitcoin,sipsorcery/bitcoin,anditto/bitcoin,lateminer/bitcoin,AkioNak/bitcoin,pataquets/namecoin-core,dscotese/bitcoin,jambolo/bitcoin,namecoin/namecore,Xekyo/bitcoin,ElementsProject/elements,fujicoin/fujicoin,yenliangl/bitcoin,namecoin/namecoin-core,jamesob/bitcoin,mm-s/bitcoin,bitcoinknots/bitcoin,pataquets/namecoin-core,andreaskern/bitcoin,dscotese/bitcoin,yenliangl/bitcoin,jambolo/bitcoin,pataquets/namecoin-core,yenliangl/bitcoin,sipsorcery/bitcoin,jamesob/bitcoin,namecoin/namecore,dscotese/bitcoin,kallewoof/bitcoin,Xekyo/bitcoin,namecoin/namecoin-core,jamesob/bitcoin,bitcoin/bitcoin,ElementsProject/elements,jlopp/statoshi,qtumproject/qtum,achow101/bitcoin,anditto/bitcoin,JeremyRubin/bitcoin,ajtowns/bitcoin,Xekyo/bitcoin,achow101/bitcoin,instagibbs/bitcoin,tecnovert/particl-core,domob1812/bitcoin,GroestlCoin/bitcoin,apoelstra/bitcoin,andreaskern/bitcoin,sstone/bitcoin,bitcoin/bitcoin,qtumproject/qtum,mruddy/bitcoin,pataquets/namecoin-core,pataquets/namecoin-core,sipsorcery/bitcoin,dscotese/bitcoin,sstone/bitcoin,fanquake/bitcoin,kallewoof/bitcoin,domob1812/bitcoin,Xekyo/bitcoin,ajtowns/bitcoin,bitcoinsSG/bitcoin,fujicoin/fujicoin,apoelstra/bitcoin,JeremyRubin/bitcoin,bitcoinsSG/bitcoin,namecoin/namecore,GroestlCoin/GroestlCoin,tecnovert/particl-core,particl/particl-core,MarcoFalke/bitcoin,ElementsProject/elements,domob1812/bitcoin,AkioNak/bitcoin,MarcoFalke/bitcoin,jambolo/bitcoin,jambolo/bitcoin,GroestlCoin/bitcoin,kallewoof/bitcoin,domob1812/bitcoin,bitcoinknots/bitcoin,achow101/bitcoin,tecnovert/particl-core,namecoin/namecore,MarcoFalke/bitcoin,tecnovert/particl-core,bitcoinknots/bitcoin,andreaskern/bitcoin,domob1812/namecore,bitcoinsSG/bitcoin,apoelstra/bitcoin,JeremyRubin/bitcoin,namecoin/namecoin-core,instagibbs/bitcoin,mm-s/bitcoin,ElementsProject/elements,JeremyRubin/bitcoin,andreaskern/bitcoin,MarcoFalke/bitcoin,prusnak/bitcoin,mruddy/bitcoin,instagibbs/bitcoin,MarcoFalke/bitcoin,mruddy/bitcoin,particl/particl-core,fujicoin/fujicoin,andreaskern/bitcoin,GroestlCoin/GroestlCoin,bitcoin/bitcoin,particl/particl-core,anditto/bitcoin,jambolo/bitcoin,particl/particl-core,domob1812/bitcoin,sstone/bitcoin,kallewoof/bitcoin,qtumproject/qtum,bitcoinknots/bitcoin,jamesob/bitcoin,bitcoinsSG/bitcoin,GroestlCoin/bitcoin,mruddy/bitcoin,bitcoinknots/bitcoin,MarcoFalke/bitcoin,jambolo/bitcoin,ajtowns/bitcoin,andreaskern/bitcoin,lateminer/bitcoin,dscotese/bitcoin,rnicoll/bitcoin,ajtowns/bitcoin,GroestlCoin/GroestlCoin,ElementsProject/elements,fanquake/bitcoin,anditto/bitcoin,prusnak/bitcoin,rnicoll/bitcoin,prusnak/bitcoin,domob1812/namecore,sipsorcery/bitcoin,GroestlCoin/GroestlCoin,JeremyRubin/bitcoin,fanquake/bitcoin,tecnovert/particl-core,prusnak/bitcoin,anditto/bitcoin,fanquake/bitcoin,JeremyRubin/bitcoin,instagibbs/bitcoin,lateminer/bitcoin,jlopp/statoshi,fanquake/bitcoin,anditto/bitcoin,fujicoin/fujicoin,fujicoin/fujicoin,AkioNak/bitcoin,prusnak/bitcoin,GroestlCoin/GroestlCoin,ElementsProject/elements,kallewoof/bitcoin,sstone/bitcoin,tecnovert/particl-core,apoelstra/bitcoin,yenliangl/bitcoin,GroestlCoin/bitcoin,Xekyo/bitcoin,bitcoin/bitcoin,mm-s/bitcoin,apoelstra/bitcoin,namecoin/namecore,yenliangl/bitcoin,bitcoin/bitcoin,bitcoinsSG/bitcoin,jlopp/statoshi,lateminer/bitcoin,fanquake/bitcoin,GroestlCoin/GroestlCoin,jamesob/bitcoin,mruddy/bitcoin