Function parse_file 

pub fn parse_file(
    sources: &SourceMap,
    file: FileId,
) -> Result<SourceUnit, Vec<Diagnostic>>

Lexes, parses, and numbers one .cove file.

Numbering happens here rather than in the parser so that every caller gets it: a unit this function returns has an ast::ExprId on every expression, and none of them is ast::ExprId::UNSET.