Function parse_scheme_source 

pub fn parse_scheme_source(
    codec: CodecId,
    source_id: SourceId,
    source: &str,
    budget: &mut DecodeBudget,
) -> Result<LocatedExprTree>

Parses one top-level Scheme form from source text into a located Expr tree.

Lower-level entry point behind decode_scheme_tree; the caller supplies the codec id, interned source id, and a DecodeBudget. Errors if the input holds more than one top-level expression.