splitting htup.h

Hi,

This patch splits htup.h in two pieces -- the first one (tupbasics.h;
not wedded to the name) does not include many other headers and is just
enough to have other parts of the code create tuples and pass them
around, to be used by most other headers.  The other one (which keeps
the name htup.h) contains internal tuple stuff (struct declarations
etc).

Before patch, htup.h is directly or indirectly included by 364 .c files
in src/backend; after patch, that's reduced to 299 files (that's 65
files less to compile if you modify the header).

-- 
Álvaro Herrera <alvherre@alvh.no-ip.org>