[fud2/flang] Implement a Parser for Flang

[jku20]

#2555 is a messy draft of a parser for a proposed syntax for flang. This is a useful thing to have designed for thinking about and writing the language as a human and it's even better (though not mission critical) for this syntax to be real.

I'm unsure if a parser for a nice flang syntax should be part of fud-core or instead be it's own separate tool in the calyx ecosystem. Either way, it would be nice for this syntax to be refined, implemented somewhere in a more readable fashion, and then documented in the calyx docs.