REORG: includes: create tinfo.h for the thread_info struct

REORG: includes: create tinfo.h for the thread_info struct

The thread_info struct is convenient to store various per-thread info
without having to resort to a painful thread_local storage which is
slow and painful to initialize.

The problem is, by having this one in thread.h it's very difficult to
add more entries there because everyone already includes thread.h so
conversely thread.h cannot reference certain types.

There's no point in having this there, instead let's create a new pair
of files, tinfo{,-t}.h, which declare the structure. This way it will
become possible to extend them with other includes and have certain
files store their own types there.