Add fuzzing harness for `SigHasLowR(...)` and `ecdsa_signature_parse_der_lax(...)`.

See [`doc/fuzzing.md`](https://github.com/bitcoin/bitcoin/blob/master/doc/fuzzing.md) for information on how to fuzz Bitcoin Core. Don't forget to contribute any coverage increasing inputs you find to the [Bitcoin Core fuzzing corpus repo](https://github.com/bitcoin-core/qa-assets).

Happy fuzzing :)

mkdir bfuzz && cd bfuzz && cmake .. -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DENABLE_SANITIZERS="fuzzer;address;undefined"
ninja fuzz-secp256k1_ec_seckey_import_export_der fuzz-secp256k1_ecdsa_signature_parse_der_lax
./src/test/fuzz/secp256k1_ec_seckey_import_export_der
./src/test/fuzz/secp256k1_ecdsa_signature_parse_der_lax
cd ..
mkdir build && cd build && cmake .. -GNinja
ninja all check check-functional