Document equality and conversion to inheritence

I have made two minor changes and a major change in the proposed code.
!. One minor change is that equal keys in stdlib_hashmap_wrappers is now exported
   as an equality operator and not under its own name.
2. Another minor chane is the procedure relative_loading has been removed.
3. The major change is that I have converted the proper hash map codes into an
   inheritence tree with the following changes.
   A. The hash map types are now defined in a new module stdlib_hashmaps with
      the obvious filename, stdlib_hashmaps.f90.
   B. There are now three hash map types: hashmap_type, chaining_hashmap_type,
      and open_hashmap_type with hashmap_type an abstract type that serves as
      the parent to the other two types.
   C. The files stdlib_hashmap_chaining.f90 and stdlib_hashmap_open.f90 now define
      the respective submodules stdlib_hashmap_chaining and stdlib_hashmap_open
      of the module stdlib_hashmaps and largely implement the codes for their
      respective types, chaining_hashmap_type and open_hashmap_type.

[ticket: X]

Author: wclodius2