Use edition-2018 style modules

Use `module_name.rs` instead of `module_name/mod.rs`.

Pros:
- It's easier to find the module file (especially for modules with lots of
  submodules like `types`)
- Module file name is not 'magical'

src/types.rs

@@ -190,5 +190,10 @@ mod passport_data;
 mod passport_element_error;
 mod passport_file;
 
-pub use non_telegram_types::*;
-mod non_telegram_types;
+pub use non_telegram_types::{country_code::*, currency::*, non_strict_vec::*};
+mod non_telegram_types {
+    pub(super) mod country_code;
+    pub(super) mod currency;
+    pub(crate) mod mime;
+    pub(super) mod non_strict_vec;
+}

src/types/non_telegram_types/mod.rs

@@ -1,9 +0,0 @@
-pub use country_code::*;
-pub use currency::*;
-pub use non_strict_vec::*;
-
-mod country_code;
-mod currency;
-mod non_strict_vec;
-
-pub(crate) mod mime;