Add get_key and get_value_type to dict_pkg

[ru551n]

Why

The vunit-python-bridge needs to enumerate a dict_t and read each value by type through public subprograms, without touching the private p_bucket_* fields.

API added (dict_pkg)

impure function get_key(dict : dict_t; index : natural) return string;
impure function get_value_type(dict : dict_t; key : string) return data_type_t;

get_key: 0 <= index < num_keys(dict); order unspecified but stable while the dict is unmodified; reports an error if out of range. get_value_type reports an error if the key is missing. dict_pkg-2008p needs no change.

Tests

New tb_dict case enumerating a dict of mixed types, checking get_key/get_value_type after remove and across a resize (200 keys). Ran with NVC: 41 of 41 pass in the data_types run.py (tb_dict). The out-of-range error path is not tested (no existing failing-check pattern in tb_dict).

A news fragment is included.

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[ru551n]

Needed by the Python dict support in VUnit/vunit-python-bridge#11, which enumerates a dict_t with get_key and reads each value by type with get_value_type.

[ru551n]

Also fixes new_dict crashing after deallocate of a dict that had grown beyond one bucket: the pointer pool recycles pointers that can be longer than requested, but dict_t uses their lengths as bucket count and size. New regression test included (it crashes without the fix).