core: Implement vtrace (part 1 of many)
core: Implement vtrace (part 1 of many)

In this CL I add a basic implementation of vtrace spans and the vtrace
store.  Also add enough functionality to collect trace data when
javascript is acting as a client.

There are many shortcomings which will be addressed by future CLs.
- The store always collects, there is no pattern based collection as in go.
- JS servers never return trace information
- There is no interface to dump traces from the console / etc.

MultiPart: 2/4
Change-Id: Ia476f4f8929889f890e43a09fb29958d0dd3daa8

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23