Deprecate and replace type_getarray_val()

In the first version of the introduce1 branch, the type_getarray_val() accessor was used erroneously in a way that could have resulted in a buffer overflow.

That bodes ill for its usability. And since Trunnel is supposed to make it hard for us to make mistakes, we should consider fixing this.

Brainstorming replacement ideas:

• type_copyinto_val(object, input, input_len);
• type_copyfrom_val(object, target, target_len);
• type_getarray_checked(object, array, need_len);