Added a procedure for getting all the keys in a hashmap

[degawa]

To add a procedure for getting all the keys in a hashmap, the subroutine get_all_keys is added to the hashmap types.

get_all_keys is implemented as a subroutine rather than a function to maintain consistency with the other procedures, especially get_other_data.

The tasks done are summarized as follows:

• Added a deferred procedure to hashmap_type
• Implemented the type-bound procedure for chaining_hashmap_type and open_hashmap_type.
• Added some tests and an example.
• Updated the specification document for hashmaps.

closes #732

[jvdp1]

Thank you @degawa for this PR. LGTM! I have only a few minor suggestions.

[jvdp1]

Should it be "present" or "presented"?

[degawa]

I use "presented in" to mean "located in," "existing in," or "contained in" here.
I cannot answer whether "present" or "presented" is correct due to my poor English skills, so I need help to create a natural expression, especially a phrase referring to "keys in a hash map."

[jvdp1]

@milancurcic Any idea how it should be written?

[degawa]

@jvdp1
The following expression is used in the documentation of Map in Java:

keySet()
Returns a Set view of the keys contained in this map.

So, I would like to change "presented" to "contained" to follow this expression. I would appreciate your comments.
Thank you.

[jvdp1]

@degawa "contained" sounds good to me. Please, go ahead!

[degawa]

Thank you, @jvdp1, for giving me a supportive push forward.
I fixed the documentation for the get_all_keys and pushed it.
I would appreciate a review.

[jvdp1]

LGTM. Thank you for this PR @degawa !