Merge bitcoin/bitcoin#30742: kernel: Use spans instead of vectors for passing block headers to validation functions

a2955f0979 validation: Use span for ImportBlocks paths (TheCharlatan)
20515ea3f5 validation: Use span for CalculateClaimedHeadersWork (TheCharlatan)
52575e96e7 validation: Use span for ProcessNewBlockHeaders (TheCharlatan)

Pull request description:

  Makes it friendlier for potential future users of the kernel library if they do not store the headers in a std::vector, but can guarantee contiguous memory.

  Take this opportunity to also change the argument of ImportBlocks previously taking a `std::vector` to a `std::span`.

ACKs for top commit:
  stickies-v:
    re-ACK a2955f0979 - no changes except further walking the ~file~ path of modernizing variable names.
  maflcko:
    ACK a2955f0979 🕑
  achow101:
    ACK a2955f0979
  danielabrozzoni:
    ACK a2955f0979

Tree-SHA512: 8b07f4ad26e270b65600d1968cd78847b85caca5bfbb83fd9860389f26656b1d9a40b85e0990339f50403d18cedcd2456990054f3b8b0bedce943e50222d2709

src/rpc/mining.cpp

@@ -1096,7 +1096,7 @@ static RPCHelpMan submitheader()
     }
 
     BlockValidationState state;
-    chainman.ProcessNewBlockHeaders({h}, /*min_pow_checked=*/true, state);
+    chainman.ProcessNewBlockHeaders({{h}}, /*min_pow_checked=*/true, state);
     if (state.IsValid()) return UniValue::VNULL;
     if (state.IsError()) {
         throw JSONRPCError(RPC_VERIFY_ERROR, state.ToString());