S3: Support signature=anonymous to access a S3 bucket anonymously

This can be used, for example, with importtree=yes to import from a public
bucket.

This needs a patch that has not yet landed in the aws library, and will
need to be adjusted to support compiling with old versions of the library,
so is not yet suitable for merging.
See https://github.com/aristidb/aws/pull/281

The stack.yaml changes are provided to show how to build against the aws
fork and will need to be reverted as well.

Sponsored-by: Dartmouth College's DANDI project

stack.yaml

@@ -11,10 +11,10 @@ flags:
     gitlfs: true
 packages:
 - '.'
-resolver: lts-18.13
+resolver: lts-19.16
 extra-deps:
 - IfElse-0.85
-- aws-0.22
+- /home/joey/tmp/aws
 - bloomfilter-2.0.1.0
 - git-lfs-1.2.0
 - http-client-restricted-0.0.4
@@ -24,4 +24,3 @@ extra-deps:
 - base16-bytestring-0.1.1.7
 - base64-bytestring-1.0.0.3
 - bencode-0.6.1.1
-- http-client-0.7.9