Tim Duesterhus 35005d01d2 MINOR: ist: Add struct ist istalloc(size_t) and void istfree(struct ist*)
`istalloc` allocates memory and returns an `ist` with the size `0` that points
to this allocation.

`istfree` frees the pointed memory and clears the pointer.
2020-03-05 19:52:07 +01:00
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