The macro 'checkstackp' can run a GC step and destroy a preallocated
CallInfo. It has been renamed to 'checkstackGCp' to hint of this
collateral effect.

Macro 'checkstackGC' did a GC after resizing the stack; this GC can
shrink back the stack, undoing the work of 'checkstackGC'.

Allow memory errors to be raised through the API (throwing the
error with the memory error message); error in external allocations
raises a memory error; memory errors in coroutines are re-raised
as memory errors.

To allow their use in memory tests, some functions in 'ltests.c'
should never allocate memory. To avoid this allocation, the
library registers the strings used for status codes, and keeps
the variable '_WARN' always defined (with false instead of nil).

Comments in makefile and function 'l_str2d'.

Several of its callers needed that case and had to do the check
themselves.

The function 'lua_pushfstring' is seldom called with large strings,
there is no need to optimize too much for that cases.

In 'lundump.c', when loading the upvalues of a function, there can be
a read error if the chunk is truncated. In that case, the creation
of the error message can trigger an emergency collection while the
prototype is still anchored. So, the prototype must be GC consistent
before loading the upvales, which implies that it the 'name' fields
must be filled with NULL before the reading.

Added as incompatibility, in the manual, the extra return of 'io.lines'.