a.k.a lists of maps
Implementation was pretty straightforward - I had to tweak the `needsComma`
handling since it was stuck inside literal parsing. It happens out front
now. I also promoted the `assign_deep.hcl` parser test to a decoder
test that passes, since it was testing for an error to occur but now it
works! :)
Additionally we make ObjectLists support being comma-delimited, which
enables maps to defined inline like `{one = 1, two = 2}`.
Refs https://github.com/hashicorp/terraform/issues/7142
