tracing: add support for using multiple tracers

[tsandall]

The current topdown.Query interface only allows callers to supply one tracer at a time. This is fine for simple cases but it has some limitations. For example, if a user wants to enable tracing and profiling, they can't.

We could extend topdown.Query (and rego.Rego) to support multiple tracers by maintaining a set of tracers in the implementation instead of a single one. For example:

r := rego.New(rego.Tracer(tracer1), rego.Tracer(tracer2), ...)
r.Eval(ctx)
// do something with tracer1
// do something with tracer2

The same applies to topdown.Query:

q := topdown.NewQuery(ast.MustParseQuery("true")).WithTracer(tracer1).WithTracer(tracer2)
rs, err := q.Run()
// do something with tracer1
// do something with tracer2

This could be used immediately in the REPL as well as the eval and test subcommands where profiling and coverage may be used in conjunction with tracing (respectively).

[tsandall]

This was fixed in f79b301