Patch to remove auto-casting between numbers and strings

[Reuben Thomas <rrt@...>]

I have worked up a patch (against 5.1.1) that removes auto-casting 
between numbers and strings. It currently works in most of the major 
cases (arithmetic, concatenation, #) although there's still some work to 
do in the libraries.

The reason I'm posting about it is to ask if anyone else is interested. 
Specifically:

1. Would anyone like the patch? If so, I'd check the library cases a bit 
more carefully and document what I did.

2. Would it be interesting enough to make it a compile-time option? From 
looking at the code it would seem possible to replace internal calls to 
various tonumber and tostring options with "casttonumber" and 
"casttostring", which could have appropriate compile-time definitions.

Motivation for the patch: I don't like automatic casting; it hides bugs. 
I also think it rarely[1] helps users, and if it does (e.g. in a config 
file), then you can do the auto-casting yourself (e.g. read a config 
file, and call "to<type>(v)" on each value you read).

[1]For example, print is one function I've left alone. This is an 
example of the fine line in Lua between auto-casting and intentionally 
calling a tostring method.

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