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).

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.

The parser were mixing compiler indices of variables with stack indices,
so that when a to-be-closed variable was used inside the scope of
compile-time constants (which may be optimized away), it might be closed
in the wrong place. (See new tests for examples.)

Besides fixing the bugs, this commit also changed comments and variable
names to avoid that kind of confusion and added tests.

(Undoing part of commit f53eabeed8.)  It is better to keep this encoding
stable, so that all Lua versions can read at least the version of a
binary file.

The string "(null)" used for non-collectable values must be printed as a
string, not as a pointer. (Bug introduced in commit e0cbaa50fa7).

Several details in code (e.g., moving a variable to the most inner
scope that encloses its uses), comments, parameter names, extra tests.