Add negative hexstring support to getNumber.

src.ts/utils/maths.ts

@@ -76,6 +76,9 @@ export function getBigInt(value: BigNumberish, name?: string): bigint {
             return BigInt(value);
         case "string":
             try {
+                if (value[0] === "-" && value[1] !== "-") {
+                    return -BigInt(value.substring(1));
+                }
                 return BigInt(value);
             } catch(e: any) {
                 throwArgumentError(`invalid BigNumberish string: ${ e.message }`, name || "value", value);