Better variable handling

[jacobneu]

Changes

• Make the GAT type a structure (instead of just an alias for Con), storing the default names and implicit/explicit/anonymous markings from the original GAT declaration.
• Remove namedGAT, since this info is now stored in the GAT type.
• Make Alg and DAlg printing default to the names stored in the GAT if none are supplied (keeping genNames backup, in case the GAT contains insufficient names
• Closes Ability to mark arguments as implicit, and have the parser accept when they are not supplied at application #5: allows variables to be marked explicit or implicit. If implicit, then they are not (and cannot be) supplied at application
• Closes Fail if too many or two few arguments are supplied, or if implicit arguments are given explicitly #9: Tracks argument number and fails if too few or too many arguments are supplied (including if implicit args are supplied explicitly)

Remaining Tasks

• Syntax for explicitly supplying implicit arguments #6
• Inference of implicit arguments, so that resulting signature is correct #7
• Fail if argument is of the wrong type #10