Use SCC algorithm to handle (mutually) recursive types

[goffrie]

This changes both set_boxed_if_recursive and calc_impls to calculate strongly connected components, instead of using a naive recursive DFS.

• This makes the set_boxed_if_recursive calculation linear in the size of the input (instead of needing to traverse the type graph once for each message type). Also, we no longer box recursive fields if there is already a box_it annotation elsewhere that would break the cycle (see MutuallyRecursiveWithBoxedB).
• For calc_impls this makes the analysis more precise instead of always falling back to !Eq + !Copy when encountering a cycle; e.g. RecursiveOneof now derives Eq etc.