More reified utilities

[poteat]

The most interesting part of this pull request is an optimization I had to perform for lazyPipe - for some indiscernible reason, this improves performance for pipes of complex operations - my theory is that it's somehow deferring evaluation, or improving what types can be cached.

• Reify NaturalNumber.Add to a value-level function.
• Add Kind.JuxtN to juxt kinds with an arity of greater than one.
• Add NaturalNumber.FromHex to convert a hexadecimal string to a decimal number.
• Add NaturalNumber.ToHex to convert a decimal number to a hexadecimal string.
• Reify String.FromList to a value-level function.
• Add List.PadStart to pad a list to a desired length with a padding value.
• Add List.PadEnd to pad a list to a desired length with a padding value.
• Add String.PadStart to pad a string to a desired length with a padding character.
• Add String.PadEnd to pad a string to a desired length with a padding character.
• Improve inference efficiency of List.Collate and Kind.LazyPipe.