Fix and test signed/unsigned comparison semantics

There's the classical C footgun:
```c
int x = -1;
unsigned int y = 1;
if (x < y) { ... } // branch NOT taken — x converts to UINT_MAX
```
We're currently subject to this behavior, which isn't consistent with Python semantics (which we otherwise try to follow). Tweak `map_comparison` in `ExpressionToCExpressionMapper` in `loopy/target/c/codegen/expression.py` to treat mixed-signedness comparisons correctly. Add exhaustive tests. Also cover mixed-signedness subtraction.