WIP for new assign_budget() using topographical ordering
WIP for new assign_budget() using topographical ordering

SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr