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.

String literal expressions have their own kind VKSTR, instead of the
generic VK. This allows strings to "cross" functions without entering
their constant tables (e.g., if they are used only by some nested
function).

Local constant variables initialized with compile-time constants
are optimized away from the code.

This commit detaches the number of active variables from the
number of variables in the stack, during compilation. Soon,
compile-time constants will be propagated and therefore will
not exist during run time (in the stack).

VLOCAL expressions keep a reference to their corresponding 'Vardesc',
and 'Upvaldesc' (for upvalues) has a field 'ro' (read-only). So, it is
easier to check whether a variable is read-only. The decoupling in
VLOCAL between 'vidx' ('Vardesc' index) and 'sidx' (stack index)
should also help the forthcoming implementation of compile-time
constant propagation.

Removed the field 'name' from the structure 'Vardesc', as the name
of the local variable is already available in the prototype of the
function, through the index 'idx'.

A variable can be declared const, which means it cannot be assigned to,
with the syntax 'local <const> name = exp'.

  comments and other janitorial work.