Consider using signatures on function declarations

[thelonious]

Right now there is no type info because we coerce to the types we need at runtime. However, having type info to show intention is useful for documentation purposes. Whatever syntax we use here, it should be optional. It should also capture the concept of stack before/after states. In other words, the arguments represent values that need to be on the stack before the invocation, but we should also show what will be left on the stack. That will be zero or more items