(Undoing part of commit f53eabeed8.)  It is better to keep this encoding
stable, so that all Lua versions can read at least the version of a
binary file.

- function names start with lower case;
- state is always the first parameter.

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.)

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

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.

- LUAC_VERSION is equal to LUA_VERSION_NUM, and it is stored
as an int.

- 'sizeof(int)' and 'sizeof(size_t)' removed from the header, as
the binary format does not depend on these sizes. (It uses its
own serialization for unsigned integer values.)

Version numbers and dates (mostly wrong) from RCS keyword strings
removed from all source files; only the file name are kept.

rttype -> rawtt; ttyperaw -> withvariant; ttype -> ttypetag;
tnov -> ttype