Optimise processTopology()

[bboreham]

From a memory profile, 11% of all space is allocated in process.reporter.processTopology(); this PR creates a benchmark and reduces allocations by half through calling different APIs.

Also optimised the merge-to-empty case in a few sub-structures that should help a little elsewhere.

before:

BenchmarkReporter-2   	   10000	    105043 ns/op	   32205 B/op	     227 allocs/op

after:

BenchmarkReporter-2   	   50000	     34800 ns/op	   15818 B/op	     161 allocs/op