vendor: add github.com/a8m/tree

vendor/github.com/a8m/tree/compileall.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+go tool dist list >/dev/null || {
+    echo 1>&2 "go tool dist list not supported - can't check compile"
+    exit 0
+}
+
+while read -r line; do
+    parts=(${line//\// })
+    export GOOS=${parts[0]}
+    export GOARCH=${parts[1]}
+    echo Try GOOS=${GOOS} GOARCH=${GOARCH}
+    go install
+done < <(go tool dist list)