Improve probabilistic program definition syntax

[marcoct]

If possible, change the current syntax:

@program foo(args..) begin
    .. body
end

to:

@probabilistic function foo(args...)
    .. body
end

This syntax be more familiar to Julia users, and would make it clearer that what they're defining is very much like a Julia function, which is defined like:

function foo(args...)
    .. body
end