The cfg subpackage builds a control flow graph of ast.Nodes.
The lostcancel module checks this graph to find paths, from a call to
WithCancel to a return statement, on which the cancel variable is
not used.  (A trivial case is simply assigning the cancel result to
the blank identifier.)

In a sample of 50,000 source files, the new check found 2068 blank
assignments and 118 return-before-cancel errors.  I manually inspected
20 of the latter and didn't find a single false positive among them.

Change-Id: I84cd49445f9f8d04908b04881eb1496a96611205
Reviewed-on: https://go-review.googlesource.com/24150Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Rob Pike <r@golang.org>