Variants like dictionaries and arrays can have cyclic references, which
caused `encode_variant` to run an infinite recursion.
Instead of keeping a stack and looking for cyclic references which would
make serialization slower, this commit adds a `MAX_RECURSION_DEPTH`
constant to Variant, and have `encode_variant` keep track of the current
recursion depth, bailing when it's too high since this likely means a
cyclic reference has been encountered.
Fabio Alessandrelli
324636473a