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.

It is simpler to signal a to-be-closed upvalue with a boolean flag,
instead of using a different tag.

- Macro 'checkliveness' (for debug) always uses 'L', to avoid warnings.
- Some old 'while' changed to 'for' in 'testes/gc.lua'.
- In 'testes/libs/makefile', do not make files depend on 'ltests.h',
which may not even exist.