Update Go versions used in travis and LANGUAGES.md

Per our support policy, drop support of go 1.16 and add support of go
1.18.

Also enable go vet in make check, and fix issues reported by go vet.
diff --git a/lib/go/test/fuzz/Makefile.am b/lib/go/test/fuzz/Makefile.am
index 391c84a..a8a8102 100644
--- a/lib/go/test/fuzz/Makefile.am
+++ b/lib/go/test/fuzz/Makefile.am
@@ -33,4 +33,4 @@
 	fuzz.go \
 	fuzz_test.go \
 	go.mod \
-	go.sum 
+	go.sum