Macro ascii_alu

macro_rules! ascii_alu {
    ($name:ident,
     // safety invariant: src/dst MUST be u8
     $src_unit:ty,
     $dst_unit:ty,
     // Safety invariant: stride_fn must consume and produce two usizes, and return the index of the first non-ascii when it fails
     $stride_fn:ident) => { ... };
}