Function monomorphize 

pub fn monomorphize<'a>(
    stmts: &[Stmt<'a>],
    registry: &TypeRegistry,
    interner: &mut Interner,
    arenas: Arenas<'a>,
    options: Options,
) -> Result<Monomorphized<'a>, String>

Rewrite stmts into a program with no generic function, no applied generic type, and (under options.lower_option) no built-in Option.