Change-Id: I6fa9f1b65d6c06b891aabf3f3d868364d74d727d

Fixes #1029

The sources themselves appear to already be Python 3 clean.

This changes csharp_names.h, which will require a corresponding change in GRPC.

This adds implicit dependencies to the coressponding runtime libraries.

We need to use the list to indicate field presense.
The field must only contain 0 or 1 string element.

This is needed to support python bazel build.

library.

When trying to compile the protobuf code as a DLL, and then compile other DLLs
with generated .pb.cc/h files that reference
InternalMetadataWithArena::InternalMetadataWithArena(Arena*), MSVC gives an
"unresolved external symbol" error.  This seems to be due to the function being
simultaneously exported and inline.  Moving it out-of-line fixes things.

There are other functions exported and inline as well but de-inlining them
doesn't seem to be necessary to get the build working, and I'd rather de-inline
as few functions as possible.

Change-Id: I17e16fdae6e4d2fb74d178fa5564a609ed58af1d

Also add WKT headers to the umbrella file, and simplify Podspec with it.
Plus some layout improvements to the BUILD file.

Change-Id: I63ed5fb58a45e098f2fd1fa457a219148de030bd

Induce those from the input proto file names instead to reduce the
lenght of the BUILD file.

Change-Id: I1dda57dedac547f4bd0cc3a7d08fb611d72acd13

Change-Id: I0b5a2c017f5412aaca3a47815671753cdef63165

Change-Id: I62640af99e3c2650a29dc5468ce3deb7b6c7efdf

Change-Id: I5a299d969ff96d7d2f80aadc7e8987d461d24b8f