Clarify heap types for arrays

[gmalecha-at-skylabs]

Unify the two functions:

• to_heap_type ( rocq-bluerock-brick/theories/lang/cpp/syntax/types.v), and
• heap_type_of (rocq-bluerock-brick/theories/lang/cpp/logic/initializers.v)

Further,

1. refine the notion of heap types for arrays (maybe just heap types of the element type?) to be compatible with wp_init_well_typed. The complexity here lies in Tincomplete_array and Tvariable_array.
2. prove that to_heap_type always produces a type that satisfies is_heap_type.