*: Support both Prost and rust-protobuf libraries

To use Prost, set the `PROST` env var, e.g.,: `PROST=1 make dev`. If using Cargo,
use `--no-default-features --features prost-codec`.

The most notable change is threading the prost-codec/protobuf-codec through the
Cargo.tomls of all crates. In addition, in order to make this work I had to move
integraton tests and benchmarks into their own crate (`tests`). This is because
Cargo features do not interact perfectly with dev-dependencies.

We're using a Git dep for Prost in order to get some optimisations which are on
master, but not in the latest release. We can change to a crates.io dep when there
is another release.

We must allow the `identity_conversion` lint because there are some conversions
which are meaningful with rust-protobuf, but no-ops with Prost.

The changes to src/coprocessor/endpoint.rs are because Prost does not permit
setting a custom recursion limit. We only did this for tests previously. We
now use the default recursion limit all the time for both codecs; the test must
be adjusted so that we hit the higher limit.

Signed-off-by: Nick Cameron <nrc@ncameron.org>