As it was said in the previous patch here goes gotrace utility to
process `//trace:event ...` and other tracing related directives.

Related excerpt from the documentation:

---- 8< ----
Gotrace

The way //trace:event and //trace:import works is via additional code being
generated for them. Whenever a package uses any //trace: directive,
it has to organize to run `gotrace gen` on its sources for them to work,
usually with the help of //go:generate. For example:

	package hello

	//go:generate gotrace gen .

	//trace:event ...

Besides `gotrace gen` gotrace has other subcommands also related to tracing,
for example `gotrace list` lists trace events a package provides.
---- 8< ----

Gotrace works by parsing and typechecking go sources via go/parse &
go/types and then for special comments generating corresponding
additional code that is supported by tracing runtime.