Handle user-defined inlining annotations in concrete syntax

Extend the parser to cope with Haskell-line inlining annotations. We should mirror the syntax described [here](https://wiki.haskell.org/Inlining_and_Specialisation) as much as possible.

Start by reading the linked Haskell inlining and specialisation syntax description, then locate the parser entry point for concrete syntax in this repository. The work is complete when user-defined inlining annotations are accepted and handled in a way that mirrors the documented syntax as closely as possible.

Written by the indexing model from the issue text.