add sptree compute things to cakeml_computeLib

ideally this should be in an sptree library providing a compset fragment
add compile_top to cakeml_computeLib

since this is another top-level entry-point, add it with the conversion
that adds the generated code to the good code labels database (as is
done for compile_prog too).
update compile_repl_decs to assume repl_decs is in a module