Use json-like structure for metadata instead of association list.

- `Text.Pandoc.Definition`:
    + Removed `getMeta` and `getMetas`.
    + Export `MetaValue`, `nullMeta`, `lookupMeta`.
    + `MetaValue` can be pandoc blocks, a raw string,
      or a list or map of other `MetaValue`s.
- `Text.Pandoc.Builder`:
    + Removed `ToBlocks` type class.
    + Added `ToMetaValue` type class.
    + Removed `setMetas`.  `setMeta` can now be called with a list
      of values.