Fixed the bug that a key removed from a table might not be found
again by 'next'. (This is needed to allow keys to be removed during a
traversal.) This bug was introduced in commit 73ec04fc.

Tables were using this bit to indicate their array sizes were real
('isrealasize'), but this bit can be useful for tests. Instead, they
can use bit 7 of their 'flag' field for that purpose. (There are only
six fast-access metamethods.) This 'flag' field only exists in tables,
so this use does not affect other types.

Macro 'checkconsistency' replaced by the similar 'checkliveness".

Use a variable-sized array to store string contents at the end of a
structure 'TString', instead of raw memory.

Corrected several small details: added 'const', adjusts in tabs x
spaces, removed unused #includes and #defines, misspellings, etc.

Variants can use zero for first variant.

LUA_T* represents only types; tags (types + Variants) are represented
by LUA_V* constants.

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 old definition did one test for nil, but three tests for the all
too common booleans (and two tests for other values); this definition
does two tests for all values.