Make ABI loader private so we can use expect calls

- Why private?
  - This functionality is out of scope for our SDK.
  - It wasn't being used outside this module.
- I think that it is ok to go with the simplest solution right now. We
  could refactor this to emit an error in the future if necessary.
  - It makes sense to use `expect`/`unwrap` here because the
    `include_str!` macro ensures that the ABI JSON string is present.

src/abi/mod.rs

@@ -3,8 +3,7 @@ use std::str::FromStr;
 
 const L1_DEFAULT_BRIDGE_INTERFACE: &str = include_str!("./IL1Bridge.json");
 
-// FIXME this was taken from
-pub fn load_contract(raw_abi_string: &str) -> Contract {
+fn load_contract(raw_abi_string: &str) -> Contract {
     // Note that using `.expect` here is acceptable because we expect the value of
     // `L1_DEFAULT_BRIDGE_INTERFACE` to be correct. In the future, we should refactor this piece of
     // code to run in compile time.