cty: Optimize equality check for numbers of differing signs

This improves performance when comparing large numbers to zero, along
with comparing large numbers of different signs. Without this special case,
the numbers are both converted to text, and string compared, which is
costly if the numbers are large.

This provides a cheap early-exit path if the signs don't match,
speeding up the cost of comparison to zero. As part of this, the .Modulo
function calling .RawEquals should also have significantly improved
performance (which was otherwise an expensive check for large numbers).

cty/primitive_type.go

@@ -74,6 +74,8 @@ func rawNumberEqual(a, b *big.Float) bool {
 		return false
 	case a == nil: // b == nil too then, due to previous case
 		return true
+	case a.Sign() != b.Sign():
+		return false
 	default:
 		// This format and precision matches that used by cty/json.Marshal,
 		// and thus achieves our definition of "two numbers are equal if