Investigate deeply nested message reuse

[danburkert]

Reddit user nwydo points out that prost doesn't do as good of a job reusing deeply nested message allocations as the reference C++ implementation, due to differences between Vec and RepeatedPtrField. While I think this isn't necessarily as critical for prost since nested message fields are inline (which often has the effect of flattening the hierarchy), it could be significant for encoding/decoding n-ary tree-like structures. Should be relatively easy to write a benchmark for it.

One possible solution is to add a clear+merge all-in-one operation to Message (perhaps called Message::decode_into), which could re-use existing deeply nested allocations.