Bug 259: Error on unsafe signed/unsigned comparisons.

[ckamm]

Fixes http://d.puremagic.com/issues/show_bug.cgi?id=259 in a way that uses the range propagation of expressions. It is also still backwards compatible with C code, though some C code may now produce an error. It still breaks lots of existing code though.

Examples:

uint a = 3;
if (a > 4) {} // ok

uint b = 3;
if (b > -2) {} // now an error

byte c = -67;
uint u;
if (u < c + 0x100) {} // ok

int[] a;
for (int i = 0; i < a.length; ++i) {} // error, ugh

[WalterBright]

This is clever and I like it. But it does break a lot of code left and right, and perhaps that code is broken. I'm going to leave it in #ifdef'd out for the moment.

[denis-sh]

I'm going to leave it in #ifdef'd out for the moment.

#ifdef'd code now completely removed from expression.c in 8f7b3b0 (see diff)