Storage service and client (#300)

A very rudimentary storage service for archives larger than what can
fit in TPR/CRD resources. The library used allows for using local
files, AWS S3, or other cloud storage APIs. For now only local paths
are set up.

The storage service doesn't know anything about functions or packages;
it can be used for arbitrary archives.

There's no CLI integration yet.

environments/fetcher/cmd/build.sh

@@ -1,2 +1,2 @@
 #!/bin/sh
-GOOS=linux GOARCH=386 go build -o fetcher .
+CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -o fetcher .