Version of generic parser, with example.

Still need to figure out how to get the interaction between coercion
application and conversion right.
Make coercions work without notations.
Remove Eval compute in definition.
Decouple reading from result extraction.
Add list parser example, move into module.
Use coercion to read nats instead of notation.
Add friendlier syntax for parsers.
Change parser naming.
Simplify definition of reader.
Make tokens depend on current state.
Add post-fix expressions.
Remove continuation from list example.
Factor operations into their own type.
Start working on text.

I think the coinductive stuff won't be needed after all.
Replace coinductive type with an inductive one.
Progress on writing for parser definition.
Simplify state for prefix expressions.
Small tweaks, text on arithmetic expressions.
Include simplified example first.
Explain use of dependent types.