sstables: Obtain the key from entries using get_key() rather than casting to bytes_view

The entry contains not only the key, but other stuff like
position. Why would casting to bytes_view give the view on just the
key and not the whole entry. Better to be explicit.

sstables/partition.cc

@@ -33,7 +33,7 @@ namespace sstables {
  * the extra information when not needed.
  *
  * This code should work in all kinds of vectors in whose's elements is possible to aquire
- * a key view.
+ * a key view via get_key().
  */
 template <typename T>
 int sstable::binary_search(const T& entries, const key& sk, const dht::token& token) {
@@ -47,8 +47,7 @@ int sstable::binary_search(const T& entries, const key& sk, const dht::token& to
         // creation by keeping only a key view, and then manually carrying out
         // both parts of the comparison ourselves.
         mid = low + ((high - low) >> 1);
-        auto mid_bytes = bytes_view(entries[mid]);
-        auto mid_key = key_view(mid_bytes);
+        key_view mid_key = entries[mid].get_key();
         auto mid_token = partitioner.get_token(mid_key);
 
         if (token == mid_token) {

sstables/types.hh

@@ -12,6 +12,7 @@
 #include "streaming_histogram.hh"
 #include "estimated_histogram.hh"
 #include "column_name_helper.hh"
+#include "sstables/key.hh"
 #include <vector>
 #include <unordered_map>
 #include <type_traits>
@@ -43,8 +44,8 @@ struct index_entry {
     uint64_t position;
     disk_string<uint32_t> promoted_index;
 
-    explicit operator bytes_view() const {
-        return bytes_view(key);
+    key_view get_key() const {
+        return { bytes_view(key) };
     }
 };
 
@@ -52,8 +53,8 @@ struct summary_entry {
     bytes key;
     uint64_t position;
 
-    explicit operator bytes_view() const {
-        return key;
+    key_view get_key() const {
+        return { key };
     }
 
     bool operator==(const summary_entry& x) const {