move most of Dtype case of compile_dec_val into separate theorem

hoping to reuse this in something about the effect of compile_decs on
the compiler's constructor map... but may be difficult to ignore the
intervening non-Dtype declarations...
return compiler state for failed module closer to semantics

have a separate function for pulling out the effect of just the Dtype
definitions and use that to build the error next state. this should
hopefully make the correctness proof easier...
make compiler tests build again