Add time constants

[Ben-PH]

I have Use cases where having direct access to U3600 is needed. I tried making a type U3600 = op!(U36 * 100);, but when I try implementing for something that looks like this, I get an error:

trait SpecializedPeriod {}
impl SpecializedPeriod for Period<op!(U36 * U100), U1> {}
impl SpecializedPeriod for Period<U60, U1> {}
impl SpecializedPeriod for Period<U1, U1> {}

error[E0119]: conflicting implementations of trait `SpecializedPeriod` for type `period::Period<UInt<UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B1>, B1>, B1>, B0>, B0>, UInt<UTerm, B1>>`
  --> src/duration/period.rs:15:1
   |
14 | impl SpecializedPeriod for Period<op!(U36 * U100), U1> {}
   | ------------------------------------------------------ first implementation here
15 | impl SpecializedPeriod for Period<U60, U1> {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `period::Period<UInt<UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B1>, B1>, B1>, B0>, B0>, UInt<UTerm, B1>>`

error[E0119]: conflicting implementations of trait `SpecializedPeriod` for type `period::Period<UInt<UTerm, B1>, UInt<UTerm, B1>>`
  --> src/duration/period.rs:16:1
   |
14 | impl SpecializedPeriod for Period<op!(U36 * U100), U1> {}
   | ------------------------------------------------------ first implementation here
15 | impl SpecializedPeriod for Period<U60, U1> {}
16 | impl SpecializedPeriod for Period<U1, U1> {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `period::Period<UInt<UTerm, B1>, UInt<UTerm, B1>>`

This allows for such numbers to be used directly.

A follow-up issue might be to setup some way to provide specific numbers in a config file if needed, and provide these vals to the build script somehow, but that's well out of scope for this PR.