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.)

In 'aux_rawset', top must be decremented after the barrier, which
refers to top-1. (Bug introduced in commit c646e57fd.)

Both 'tonumber' and 'tointeger' cannot change the out parameter when
the conversion fails.

With MMBIN instructions, there are fewer opcodes that need to update
'L->top', so that change does not seem to pay for the increased
complexity.

Instead of updating 'L->top' in every place that may call a
metamethod, the metamethod functions themselves (luaT_trybinTM and
luaT_callorderTM) correct the top. (When calling metamethods from
the C API, however, the callers must preserve 'L->top'.)

In 'lua_settop', avoid calling 'luaF_close' when increasing the stack
or when the function has no to-be-closed variables.

When leaving a scope, the new stack top should be set only after
closing any upvalue, to avoid manipulating values in an "invalid"
part of the stack.