Also fixes incorrect location of `tv_dict_add` function and three bugs in other
functions:
1. `tv_dict_add_list` may free list it does not own (vim/vim#1555).
2. `tv_dict_add_dict` may free dictionary it does not own (vim/vim#1555).
3. `tv_dict_add_dict` ignores `key_len` argument.
Additional modifications:
- More `const` qualifiers in tested functions.
- `tv_list_find_str()` second argument is more in-line with other
`tv_list_find*()` functions.
Tests crash at some point without
- `after_each(collectgarbage)` right before “typval.c list copy() copies list
correctly and converts items” test.
- Commenting out that test.
- Adding `collectgarbage()` after the test (what actually this commit does).
Adding `collectgarbage()` to top-level `after_each` block right after
`restore_allocators` makes running this file crash even if it is run alone.
Also found some bugs:
1. var_item_copy() always fails to copy v:_null_list and v:_null_dict. Fixing
this should mean fixing `deepcopy(v:_null_list)` which should’ve been, but
was not listed in #4615. This also fixes `deepcopy(v:_null_dict)`.
2. var_item_copy() crashes when trying to copy NULL string with `conv != NULL`.
3. `conv` argument is ignored when copying list unless `deep` is true, but it
was not reflected in documentation.
4. `tv_dict_item_alloc_len()` allocated more memory then needed.
5. typvalt2lua was not able to handle self-referencing containers.
