Problem: upgrade to nom 6.0.0 fails to compile

[tomtau]

   Compiling mls v0.6.0 (/drone/src/chain-tx-enclave-next/mls)
291	error[E0277]: `?` couldn't convert the error to `keypackage::GenKeyPackageError`
292	   --> chain-tx-enclave-next/mls/src/keypackage.rs:309:55
293	    |
294	309 |         let (_, cert) = parse_x509_der(&certificate.0)?;
295	    |                                                       ^ the trait `std::convert::From<nom::internal::Err<x509_parser::error::X509Error>>` is not implemented for `keypackage::GenKeyPackageError`
296	    |
297	    = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
298	    = help: the following implementations were found:
299	              <keypackage::GenKeyPackageError as std::convert::From<nom::internal::Err<x509_parser::error::X509Error>>>
300	              <keypackage::GenKeyPackageError as std::convert::From<ra_enclave::context::EnclaveRaContextError>>
301	              <keypackage::GenKeyPackageError as std::convert::From<ring::error::KeyRejected>>
302	              <keypackage::GenKeyPackageError as std::convert::From<ring::error::Unspecified>>
303	    = note: required by `std::convert::From::from`
304	
305	error: aborting due to previous error
306	
307	For more information about this error, try `rustc --explain E0277`.
308	error: could not compile `mls`.