[01mf02]

This PR addresses problems uncovered by @dnet as part of a ROS audit for jaq's multi-format support. Thanks a lot for all your effort --- I appreciate it!
These problems are:

• XML parser panics with unwrap
• Naive allocation strategy in CBOR parser causes panic
• Non-existent anchors cause panic in YAML parser
• Arithmetic panic in handling remainder operation
• Underlying TOML parser panics for invalid inputs: This is corrected by Transition from toml-edit to toml-span. #398.
• YAML writer produces invalid YAML for certain keys
• YAML writer round-trip issue for strings starting with a dot
• YAML writer produces invalid YAML for overly long keys: There are many more such round-trip issues, e.g. for strings containing non-UTF-8 characters. We could only protect against such issues by traversing values recursively before printing them, which would incur a significant performance overhead. For that reason, we prefer outputting invalid YAML in such cases.
• Stack overflow in YAML, CBOR, JSON and XML parsers for deeply nested input: We will not be able to reliably prevent such stack overflows, because stack sizes vary between platforms. On the other hand, limiting the depth would restrict the set of values to process and require traversing the whole value to print once before printing, which would incur a significant performance overhead. We have previously considered using something like stacker, but even if we could make parsing stack-overflow-proof, basically all other parts of jaq might then start failing (Drop, printing, ...). Furthermore, stack overflows are a problem that can inherently occur in jaq scripts --- even in something like jaq -n 'def f: 1+f; f' (jq fails with an error here, too). The same problem holds for memory consumption, by the way. jaq cannot guarantee to run out of memory, and similarly, it cannot guarantee to not overflow the stack. This is now also documented in the README.

Unfortunately, these links are not public.

• BigUint::from_str_radix accepts strings with underscores as numbers