Module needs same name as struct

[Neotamandua]

When defining a contract module in a separate file and using #[contract] in lib.rs like:

#[contract]
pub mod counter

The code will not compile, if the file name is different from the struct. A mismatch between module name and struct name triggers a proc-macro panic with the following error:

proc-macro panicked: called `Option::unwrap()` on a `None` value

To Reproduce

1. Take the README example code
2. Move the module content of counter to a new file named counter.rs.
3. Rename counter.rs to contract.rs
4. Update lib.rs to use pub mod contract.