Deletion of Streams with many Chunks can trigger stack overflow

[bbannier]

The individual Chunks in a Stream are stored as a linked list. Currently deletion of a Streams with many Chunks can trigger stack overflows, see e.g., the runtime behavior caused due to #1549 (which should just have caused the parser to not trim, but never to segfault). This is due to how the default generated dtr or Chunk works: to clean up the next Chunk and to traverse the list the compiler generates a recursive call due to the definition of _next,

spicy/hilti/runtime/include/types/stream.h

Line 251 in 50e487c

std::unique_ptr<Chunk> _next = nullptr; // next chunk in chain, or null if last

We should replace this e.g., with an explicit cleanup.

[bbannier]

Also fixed in release/1.8.