all: add "protolite" build mode

[dsnet]

There's no question that the current implementation of protobufs is geared towards server environments with large amounts of memory. This issue proposes the addition of a build mode (perhaps controlled by a build tag) that optimizes for a constrained environment with low memory availability, a small footprint for binary size, or both.

Challenges:

• The dynamic creation of table-driven serializers does incur quite a bit of memory overhead. Fortunately, it's based on a per-use basis.
• Small binary footprint often runs contrary to attempts at full code-generation of the serialization logic (see protoc-gen-go: fully generate marshaling and unmarshaling code #280).
• The protobuf language has a large set of features and the runtime supports all of them. For example, a user of proto3 only does not care about extensions, but extensions support has quite a lot of complexity.
• The runtime provides a large degree of backwards compatibility with messages that have not been regenerated in a long time. This is runtime support that costs binary size and will not matter for users that have sufficiently updated messages.

\cc @neild @bradfitz

[hugelgupf]

We (LinuxBoot / u-root / go/nerf) would also really appreciate this, in terms of small binary size footprint (we have no such memory constraints). We use protos and gRPC in environments that need to compress to < 4-6MB, and gRPC and protos are one of the larger components of that. It currently works out, but I'm worried about that changing in the future.