This includes the following user-facing changes:
- Deprecated inline constructors are removed. These are `DoubleQuoted`,
`SingleQuoted`, `DisplayMath`, and `InlineMath`.
- Attr values are no longer normalized when assigned to an Inline
element property.
- It's no longer possible to access parts of Inline elements via
numerical indexes. E.g., `pandoc.Span('test')[2]` used to give
`pandoc.Str 'test'`, but yields `nil` now. This was undocumented
behavior not intended to be used in user scripts. Use named properties
instead.
- Accessing `.c` to get a JSON-like tuple of all components no longer
works. This was undocumented behavior.
- Only known properties can be set on an element value. Trying to set a
different property will now raise an error.
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