Function lower_counted_loop 

pub fn lower_counted_loop(
    source: &str,
    fn_name: &str,
    params: &[i64],
    t: &PlasticineTarget,
) -> Result<PlasticineArtifact, String>

Lower the counted loop fn_name of source onto t: one metapipeline stage under the ideal pipelined PCU model, placed on the checkerboard, VM-anchored at params, certified when its body is ALU-only, with Spatial source — or a typed reason it cannot be.