signature v3: type inference regression

[tarcieri]

It seems after #1827 type inference is not working as well:

RustCrypto/RSA#505 (comment)

error[E0282]: type annotations needed
  --> tests/pkcs1v15.rs:16:9
   |
16 |     let signature = signing_key.sign(bytes);
   |         ^^^^^^^^^
...
22 |     assert_eq!(signature.to_string(), expected);
   |                --------- type must be known at this point
   |
help: consider giving `signature` an explicit type
   |
16 |     let signature: /* Type */ = signing_key.sign(bytes);
   |                  ++++++++++++

It can no longer automatically infer the signature type in the case there is only a single non-overlapping impl, which seems pretty bad from an ergonomic perspective.