GA_DEEP_CLEAR macro for garray memory deallocation

Used to free garrays of `salitem_T` and `fromto_T` in spell.c, and
garray `wcmd_T` in ex_docmd.c.

Helped-by: Jiaqi Li
GA_DEEP_FREE_PTR: deep free macro for garrays that store simple pointers

By "simple pointer" I mean a pointer that can be freed with a call to `free`
without leaking any member pointer.

This macro does exactly what `ga_clear_strings` does.
Use GA_DEEP_CLEAR where appropriate
Merge pull request #1643 from philix/ga_deep_clear

GA_DEEP_CLEAR macro for garray memory deallocation