Avoid sub/gsub rebuilding $0 when no substitutions made (fix gsubtst7)

[benhoyt]

This change also updates stringer to a later version, as the old one doesn't work with recent Go -- the error was:

$ go generate ./...
# golang.org/x/tools/internal/tokeninternal
../../../../go/pkg/mod/golang.org/x/tools@v0.10.0/internal/tokeninternal/tokeninternal.go:78:9: invalid array length -delta * delta (constant -256 of type int64)
internal/compiler/opcodes.go:3: running "go": exit status 1