Raise a warning in case of useless comparison due to type limits

[vasily-kirichenko]

This is how it looks in Rust when you try to compare a variable of usize type (unsigned) to zero:

F# always generates https://msdn.microsoft.com/library/system.reflection.emit.opcodes.bge_un_s.aspx for uint32 comparisons.

I suggest to raise a warning in

• x < 0u
• x < 0U
• 0u > x
• 0U > x