Durations's Arbitrary instance is dependant on Gen's size

According to `Gen::new` doc:

> The size parameter controls the size of random values generated. For example, it specifies the maximum length of a randomly generated vector, but is and should not be used to control the range of a randomly generated number. (Unless that number is used to control the size of a data structure.)

However Duration's Arbitrary is defined like this:
https://github.com/BurntSushi/quickcheck/blob/aa968a94650b5d4d572c4ef581a7f5eb259aa0d2/src/arbitrary.rs#L1068-L1072

The way `seconds` is generated seems in violation of the above rule to me and has the consequence of generating very short duration only (with the default size of `100`).

According to https://doc.rust-lang.org/nightly/core/time/struct.Duration.html#method.new it seems that we should be able to use the full `u64` range without issue for seconds since we are already capping the nano seconds to one billion.

However from my testing this break `SystemTime`'s instance because we can get overflows in the following implementation:
https://github.com/BurntSushi/quickcheck/blob/aa968a94650b5d4d572c4ef581a7f5eb259aa0d2/src/arbitrary.rs#L1104-L1112

Any idea how we should fix this?