We still need access to a 16-bit system to correctly test
these changes.

Comments and small improvements in the manual.

Added documentation and asserts that constants for arithmetic opcodes
must be numbers.

- After converting a generic GCObject to a specific type ('gco2*'),
  avoid using the original GCObject (to reduce aliasing).
- Small corrections in comments in 'lopcodes.h'
- Added tests about who calls __close metamethods

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

'simplesect' encloses the introductory text of sections with
subsections, so that each section either is all text or is all
subsections. (This commit also corrects a small brace error in the
manual and extra spaces/tabs in some other files.)

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.)