split TOAST support out of postgres.h

Most backend code doesn't actually need the variable-length data types
support (TOAST support) in postgres.h. So I figured we could try to put
it into a separate header file. That makes postgres.h more manageable,
and it avoids including a bunch of complicated unused stuff everywhere.
I picked "varatt.h" as the name. Then we could either

1) Include varatt.h in postgres.h, similar to elog.h and palloc.h. That
way we clean up the files a bit but don't change any external interfaces.

2) Just let everyone who needs it include the new file.

3) Compromise: You can avoid most "damage" by having fmgr.h include
varatt.h. That satisfies most data types and extension code. That way,
there are only a few places that need an explicit include of varatt.h.

I went with the last option in my patch.

Thoughts?