let Cursor::peek return span

Summary:
Currently, `CursorBase::peek()` returns `pair<uint8_t const*, size_t>` and `CursorBase::peekView()` returns `basic_string_view<uint8_t>`. Consolidate them into `CursorBase::peek()` returning `span<uint8_t const>`.

As one motivation, libc++ v19 removes `basic_string_view<uint8_t>`.

Reviewed By: thevinster

Differential Revision: D62460851

fbshipit-source-id: 72d567817faf778287594c10dfd7f6f379bf2446

proxygen/lib/http/codec/HTTP2Framer.cpp

@@ -270,8 +270,8 @@ ErrorCode skipPadding(Cursor& cursor, uint8_t length, bool verify) {
   if (verify) {
     while (length > 0) {
       auto cur = cursor.peek();
-      uint8_t toCmp = std::min<size_t>(cur.second, length);
-      if (memcmp(cur.first, kZeroPad, toCmp)) {
+      uint8_t toCmp = std::min<size_t>(cur.size(), length);
+      if (memcmp(cur.data(), kZeroPad, toCmp) != 0) {
         return ErrorCode::PROTOCOL_ERROR;
       }
       cursor.skip(toCmp);

proxygen/lib/http/codec/compress/experimental/interop/QPACKInterop.cpp

@@ -323,7 +323,7 @@ struct QIFReader : public Reader {
         case LINESTART: {
           seenR = false;
           auto p = c.peek();
-          switch (p.first[0]) {
+          switch (p[0]) {
             case '#':
               state_ = COMMENT;
               break;
@@ -348,12 +348,12 @@ struct QIFReader : public Reader {
           break;
         case EOL: {
           auto p = c.peek();
-          if (p.first[0] == '\n') {
+          if (p[0] == '\n') {
             c.skip(1);
             state_ = LINESTART;
           } else if (seenR) { // \r followed by anything but \n -> mac newline
             state_ = LINESTART;
-          } else if (p.first[0] == '\r') {
+          } else if (p[0] == '\r') {
             c.skip(1);
             seenR = true;
           }