Support max Read/Write nesting depth limits (`StreamReadConstraints`/`StreamWriteConstraints`) for YAML

[cowtowncoder]

(note: related to / part of #430)

Currently YAML backend does not enforce any of the constraints. As the first step, let's see if we could easily add support for constraining maximum nesting depth, as this is an easily DoS-able problem.

(note: it is possible that underlying SnakeYAML still has issues with deeper nesting but we'll see)

[cowtowncoder]

Looks like writer (generator) -side limits were already enforced, but reader-side not: YAMLParser was not calling method that does validation -- but now does, for 2.17.0.