gofmt: simplify assignments of the form: v = v op y

gofmt: implemented -ss ("strong simplify") feature In contrast to gofmt -s, which enables idiomatic simplifications but leaves the programmers intentions with respect to expression of code intact, gofmt -ss is free to simplify code automatically as much as possible while preserving semantics. For instance, gofmt -s will simplify s[0:len(s)] to s[0:]; gofmt -ss will go a step further and replace s[0:b] to s[:b] (and thus s[0:len(s)] to s[:]). There is no intention in applying gofmt -ss to the basic library; this is simply a useful tool that may be applied on an individual basis. Currently implemented -ss simplifications: - v = v op y becomes: v op= y - v += 1 becomes: v++ - v -= 1 becomes: v-- - s[0:b] becomes s[:b]