Force users to call a contract!() marco at the top of the file

[richardpringle]

(alternatively, maybe we could force them to annotate lib.rs with #![contract]. Then we could do something like add a #![contract(bindings_struct = StructName)] where StructName is simply a syn::Ident.

With this we could use cfg directives to specify the #![crate_type = "cdylib"] or #![crate_type = "lib"] depending on whether we're compiling with the bindings feature (the latter for bindings).

Then we could store that struct name in global-state inside sdk-macros and write multiple impl blocks such that using the bindings as a dependency would look something like this (constructor arguments subject to change):

use counter::Counter;

let counter = Counter::new(address, external_call_context);
let val = counter.inc(alice);