Add AutoFile::detail_fread member function

New code can call the method without having first to retrieve the raw
FILE* pointer via Get().

Also, move implementation to the cpp file. Can be reviewed with:
--color-moved=dimmed-zebra  --color-moved-ws=ignore-all-space

src/streams.cpp

@@ -0,0 +1,40 @@
+// Copyright (c) 2009-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://opensource.org/license/mit/.
+
+#include <span.h>
+#include <streams.h>
+
+std::size_t AutoFile::detail_fread(Span<std::byte> dst)
+{
+    if (!m_file) throw std::ios_base::failure("AutoFile::read: file handle is nullptr");
+    return std::fread(dst.data(), 1, dst.size(), m_file);
+}
+
+void AutoFile::read(Span<std::byte> dst)
+{
+    if (detail_fread(dst) != dst.size()) {
+        throw std::ios_base::failure(feof() ? "AutoFile::read: end of file" : "AutoFile::read: fread failed");
+    }
+}
+
+void AutoFile::ignore(size_t nSize)
+{
+    if (!m_file) throw std::ios_base::failure("AutoFile::ignore: file handle is nullptr");
+    unsigned char data[4096];
+    while (nSize > 0) {
+        size_t nNow = std::min<size_t>(nSize, sizeof(data));
+        if (std::fread(data, 1, nNow, m_file) != nNow) {
+            throw std::ios_base::failure(feof() ? "AutoFile::ignore: end of file" : "AutoFile::ignore: fread failed");
+        }
+        nSize -= nNow;
+    }
+}
+
+void AutoFile::write(Span<const std::byte> src)
+{
+    if (!m_file) throw std::ios_base::failure("AutoFile::write: file handle is nullptr");
+    if (std::fwrite(src.data(), 1, src.size(), m_file) != src.size()) {
+        throw std::ios_base::failure("AutoFile::write: write failed");
+    }
+}