Performing a C-like pointer increment in Solidity

Question

I have a memory or calldata struct with an attribute of type byte[]. I want to have a memory variable with that data, except for the first n Bytes.

In C, you'd just write

typename variablename = structname.fieldname + n;

and be done. How can this be done in Solidity?

More specifically, I have a struct describing the Ethereum header:

  struct Header {
    bytes32 parentHash;
    bytes32 uncleHash;
    address coinbase;
    bytes32 root;
    bytes32 txHash;
    bytes32 receiptHash;
    byte[] bloom;
    int128 difficulty;
    int128 number;
    uint64 gasLimit;
    uint64 gasUsed;
    uint64 time;
    byte[] extra;
    bytes32 mixDigest;
    uint64 nonce;
  }

I want to verify the signatures of blocks of a proof of authority (clique consensus) chain. This signature is part of the extra field. However, there's also some other data in there which is why I need to be able to skip a variable number of bytes (the length of the extra field minus 65) and store it in a variable before using this code to extract the signature's elements:

function splitSignature(bytes memory sig)
    internal
    pure
    returns (uint8 v, bytes32 r, bytes32 s)
{
    require(sig.length == 65);

    assembly {
        // first 32 bytes, after the length prefix.
        r := mload(add(sig, 32))
        // second 32 bytes.
        s := mload(add(sig, 64))
        // final byte (first byte of the next 32 bytes).
        v := byte(0, mload(add(sig, 96)))
    }

    return (v, r, s);
}

(https://solidity.readthedocs.io/en/v0.5.12/solidity-by-example.html)

So I need to derive the last 65 Byte of the extra field to derive v, r, and s.