Rename arbiter module to verilog_axis_arbiter

[olofk]

arbiter is a reasonably common module name. Avoid namespace collisions by prefixing it with verilog_axis_

[alexforencich]

It's a sensible idea, but I don't like the prefix verilog_axis_. Yes, I suppose it is the name of the repo, but pretty much everything else in the repo has the prefix axis_. And axis_arbiter is problematic because the arbiter doesn't really have anything in particular to do with AXI stream.