Add type checking for all arguments

[orome]

Add more Haskellian type checking and validation by:

• making Message a class and requiring callers to explicitly supply a Message (using message) as an argument;
• providing a total constructor for EnigmaConfig (either using configEnigmaExcept as sketched, or typing the arguments to EnigmaConfig and passing errors to configEnigmaExcept); and
• creating and enforcing a type for valid characters for display functions (capital letters plus space; currently forced by messageChar).

Also possibly use valid character restrictions to

• make Mapping a class and enforce checks.

See:

• Calling Haskell constructors that may fail
• Validating arguments to Haskell constructors

[orome]

Decided (788dc1f) against making Message a class and requiring callers to explicitly supply a Message as an argument.

Instead Message remains a type synonym and all arguments marked of Message type, will be processed by message.

[orome]

Decided (906d40a) against enforcing a type for valid characters for display functions.

Instead, those functions just change any non-capital letters to a space.

[orome]

Leaning against a total constructor. The only scenario where it makes sense is perhaps reading configuration strings from a file.

[orome]

Settled on exposing a total constructor (in d6740b4).