Improve compatibility with makeinfo 5.1

The newer version of makeinfo is more pedantic about the use of the
@itemx token.  To allow our documentation to compile I've replaced a number
of @itemx tokens with @item tokens.

doc/reference_manual.hexi:
doc/user_guide.texi:
    As above.
Define equality for version_hash_tables.

library/version_hash_table.m:
    Add user-defined equality to the version_hash_table/2 type.

    Rename fields of the inner structure as the num_occupants function
    no-longer has the same type as the function generated by the field of
    the same name.

    We can make a similar change to hash_table.m at a later date.