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.

Instead of an explicit value (field 'b'), true and false use different
tag variants. This avoids reading an extra field and results in more
direct code. (Most code that uses booleans needs to distinguish between
true and false anyway.)

The result of 'string.format("%a", 0.0)' can have multiple zeros
after the dot.