support generating source text from HIR (to print desugared low-level fe)

[sbillig]

In some cases, we desugar high-level AST into synthetic HIR (eg a msg block becomes a module with structs and trait impls). It would be nice to print this, and even expose it via LSP (eg via a "desugar" code action on a contract block).

[sbillig]

I'll add this as part of #1183