Add new RELPATH option to address issue #271

This should allow using relative paths when importing .proto files.

See https://github.com/nanopb/nanopb/commit/cc74b9f200176edc5524aa00ba45fa90a5e87d27 for discussion.

Export examples

bazel: Add WORKSPACE

A WORKSPACE file is required for bazel to build the project. The
conventional naming is similar to Java packages with all special
characters including dots replaced by underscores.

In extra/FindNanopb.cmake, add dependency for .options files

the .options file (if it exists), so that editing the .options file will
trigger a rebuild.

Similar memset() already existed for oneofs.

pb_check_proto3_default_value() recurses into submessages,
but it didn't handle other than singular fields correctly.
This caused it to sometimes skip submessages with only
repeated or oneof fields present.

CMAKE: remove use of relative paths

When doing out of source builds:

mkdir build
cd build && cmake -G "Unix Makefiles" ../ && make

The build script tripped up on relative paths of /generator/proto and
the files under generator/proto/*.proto.

By using ${PROJECT_SOURCE_DIR}, the paths become absolute and the issue
disappears.

The test case was erroneously comparing whole submsg structures,
which could result in false errors when padding bytes differed.

Thought I was cleaning up the naming by calling the function const_cast(),
because that's what it does. But of course it conflicts with the C++ builtin,
so renaming it to pb_const_cast() now.