Simplify $blocktype function

[rossberg]

I would like to make the below simplification to the definition of the $blocktype function. However, the interpreter complaints with the following error:

8-reduction.watsup:69.9-69.44: interpreter error: BLOCK: on expr `IterE (VarE (t_2), [t_2], ^n)` invalid assignment: ?() is not an iterable value (interpreting LetI (InfixE (IterE (VarE (t_1), [t_1], ^m), ->, IterE (VarE (t_2), [t_2], ^n)), CallE (blocktype_, [ VarE (bt) ])))

@f52985, @ShinWonho, could one of you please have a look?

[ShinWonho]

rule Step_read/block:
  z; val^k (BLOCK bt instr*)  ~>  (LABEL_ n `{eps} val^k instr*)
  -- if $blocktype(z, bt) = t_1^k -> t_2^n

In the premise above, an optional value (result type of $blocktype) is assigned to an iter expression with list iter type (t_2^n).
We missed this case, so I have generalized the assigning logic to handle such cases.

[rossberg]

Excellent, thanks!