Query Profiler

[tsandall]

When policies need to be optimized by hand, the first step is to understand where time is spent in evaluating the policy. Without a query profiler this task can be quite difficult because it requires deep knowledge of the policy (which may be quite large, e.g., hundreds of lines split across multiple files.)

To start, we should add basic query profiling support to the eval subcommand.

For example:

opa eval --profile --data somedir 'data.example.allow == true'

The profiler should provide a per-expression breakdown of time spent, # of invocations, etc. The the command should also be able to sort and return the top-N expressions.

[tsandall]

Initial version added in #818 and #808