Cleaned up Axi4 types

Renamed AxiMaster* to Axi4Master*, AxiSlave* to Axi4Slave*.
Added Bit#(idWidth) id parameters where missing in the Axi4 types.
Removed unused Axi4MasterServer and mkNullAxi*Master.
Added Axi4Client and Axi4Server
updated bsvgen to use Axi4Client etc instead of AxiClient
Added Axi4Slave to enable longer read/write transactions
Use Axi4 so we can do 512 byte reads in ReadBW
reduce tracing to speed transfer
transfer 512 bytes at a time in ReadBW
write a .vcd file from the PCIe trace