Data types representing the digits zero through nine.

## Modules

Each of the following modules defines a different type named `D10`, all of which
are different representations of the same concept:

* `D10.Char` - Defines a `D10` type as a newtype for `Char`, where the values
  are restricted to characters between `'0'` and `'9'`.

* `D10.Num` - Defines a `D10` type as a newtype for any type with an instance of
  the `Num` class, where the values are restricted to numbers between `fromInteger
  0` and `fromInteger 9`.

* `D10.Safe` - Defines a `D10` type as `D0 | D1 | D2 | ... | D9`.

The `Arithmetic` modules provide infix operators `(+)`, `(-)`, `(*)` for modular
arithmetic:

* `D10.Char.Arithmetic`
* `D10.Num.Arithmetic`
* `D10.Safe.Arithmetic`

The following modules export constructors that can be used incorrectly:

* `D10.Char.Unsafe`
* `D10.Num.Unsafe`

Functions to test whether values of various types represent digits in the range
0 to 9:

* `D10.Predicate`

## Quasi-quoters

Each module that defines a `D10` type also defines quasi-quoters for it. With
the `QuasiQuotes` GHC extension enabled, a single digit like *7* can be written
as `[d10|7|]`, and a list of digits like *[4,5,6]* can be written as
`[d10|456|]`. For `D10.Char` and `D10.Num`, the quasi-quoters are an important
feature, because the `D10` types defined in these modules have unsafe
constructors, and the quasi-quoters provide compile-time assurance that we never
construct a `D10` that represents a value outside the range *0* to *9*.