Display sequences and mappings in block style

[dtolnay]

The Display impl for minimal_yaml::Yaml currently renders a map k: v as:

{"k" : "v" }

The block style would be more idiomatic for emitting maps.

k: v

[nathanwhit]

I've added the initial implementation of this in 223f6e4. It works for basic cases, but I'll continue to work on adding more tests for preservation of information and structure.