Construct high-level datatypes for C structs

[edsko]

This is the high-level counterpart to #14. For example, a field of type Ptr CChar in the low-level API might have type String the high-level API.

[edsko]

Here (and for all other high-level types, such as #40, #18) it's conceivable that we might want to use existing type instead of generating one.

[edsko]

One complication to consider here is that some structs might have multiple fields that logically are a single value, for example:

typedef struct
{
    acme_sum sum;
    acme_flag flags;
    size_t iq_len;
    acme_iq* iq_arr;
} acme_struct;

where the pair of iq_len and iq_arr is one entity, so that we might want to generate something like

data AcmeStruct = AcmeStruct {
    asSum :: AcmeSum
  , asFlags :: AcmeFlagSet
  , asIqArr :: Vector.Storable.Vector AcmeIQ
  }

(Here and elsewhere, we need to think about array ownership also.)