'coroutine.resume' did not increment counter of C calls when
continuing execution after a protected error (that is,
while running 'precover').

New module easies the inclusion of GC tracing in individual test files.

Besides errors in closing methods, 'coroutine.close' and
'lua_resetthread' also consider the original error that stopped the
thread, if any.

Shrink a stack only when the final stack size can be at most 2/3 the
previous size with half of its entries empty. This commit also
improves the clarity of 'luaD_growstack'.

- more organized handling of 'nCcalls'
- comments
- deprecation of 'setcstacklimit'

A "with stack" implementation gains too little in performance to be
worth all the noise from C-stack overflows.

This commit is almost a sketch, to test performance. There are several
pending stuff:

- review control of C-stack overflow and error messages;
- what to do with setcstacklimit;
- review comments;
- review unroll of Lua calls.

When entering a coroutine, the computation of nCcalls added 'from->nci'
to correct for preallocated CallInfos, but 'nci' includes also the
Callinfos already used.

Function names in the API use only lowercase letters.