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/configure.ac b/configure.ac
index 197d3ae..624287b 100755
--- a/configure.ac
+++ b/configure.ac
@@ -372,6 +372,7 @@
     AS_IF([test -n "$GO"],[
       ax_go_version="1.4"
       ax_go17_version="1.7"
+      ax_go118_version="1.18"
 
       AC_MSG_CHECKING([for Go version])
       golang_version=`$GO version 2>&1 | $SED -e 's/\(go \)\(version \)\(go\)\(@<:@0-9@:>@.@<:@0-9@:>@.@<:@0-9@:>@\)\(@<:@\*@:>@*\).*/\4/'`
@@ -391,6 +392,13 @@
       :
         go_version_lt_17="no"
       ])
+      AX_COMPARE_VERSION([$golang_version],[ge],[$ax_go118_version],[
+      :
+        go_version_ge_118="yes"
+      ],[
+      :
+        go_version_ge_118="no"
+      ])
     ],[
       AC_MSG_WARN([could not find Go ])
       have_go="no"
@@ -399,6 +407,7 @@
 fi
 AM_CONDITIONAL(WITH_GO, [test "$have_go" = "yes"])
 AM_CONDITIONAL([GOVERSION_LT_17], [test "$go_version_lt_17" = "yes"])
+AM_CONDITIONAL([GOVERSION_GE_118], [test "$go_version_ge_118" = "yes"])
 
 AX_THRIFT_LIB(swift, [Swift], yes)
 have_swift="no"