For Visual Studio we have to wrap the headers with push/pop pragmas
at the top and bottom of the file.

Define common macros for suppress/unsuppress KJ & the appropriate macros
for CAPNP begin/end header wrappers. Because there's a chicken egg
problem the KJ_BEGIN_HEADER/CAPNP_BEGIN_HEADER macros are placed below
all includes to ensure that the appropriate common.h file has been
sourced.