Explore refactoring Statement and Operation to be more strongly typed

[ed255]

Currently Statement and Operation are defined with a Vector of arguments that can take different types, but each operation and statement as specified takes a fixed number of arguments of fixed type each.

The devcon implementation had this typed like this:
https://github.com/0xPARC/parcnet/blob/f7ef5e2fedea10f349c6343fb24d84374edbbad2/pod2/src/pod/statement.rs#L79-L90

Explore if we can do the same in this implementation, what are the tradeoffs of each implementation, etc.

[ax0]

Closed by #69.