Add support for clearing fields by reflection

[appaquet]

Solves #627

[stepancheg]

Would you mind placing test in protobuf-test crate? With own proto file independent on descriptors.proto. Somewhere around common/v2 folder, so test would check it works for both syntax 2 and 3 (tests for v3 are copied from v2). Also if added to that folder tests will check that that it works with both protoc and pure parsers. Other than that, tests look good.

[stepancheg]

(Writing from the phone, so no links to sources.)

[appaquet]

@stepancheg updated

[stepancheg]

Published 3.1.0

[stepancheg]

Thanks!