Decode into proper arrays of things, add many test cases

/cc @armon - This changes how Consul has to define its structure. Ping
me tomorrow to learn more, but going to leave it here for reference too:

The Consul case (there is a test case) never worked even with go-libucl,
because there is an ambiguity of whether you want the inner children or
the array of outer elements (the slice in the Policy struct).

I've added a new modifier you can specify with a tag called "expand"
which will tell hcl to expand the value to its children for decoding.
You can see me use it in the test case which verifies that the Consul
ACLs parse.