libos: Make File::read_all() read chunked blocks instead of one character at a time

This results in a speedup of 0.23s -> 0.13s for Base64 encoding of /bin/ls.

libos/src/File.cpp

@@ -154,18 +154,18 @@ namespace os
 
     Result<Buffer> File::read_all()
     {
-        Vector<u8> data;
+        Buffer data;
+
+        u8 buf[2048];
 
         while (true)
         {
-            int c = TRY(getchar());
-            if (c == -1) break;
-            TRY(data.try_append((u8)c));
+            usize nread = TRY(raw_read(buf, sizeof(buf)));
+            TRY(data.append_data(buf, nread));
+            if (nread < sizeof(buf)) break;
         }
 
-        Buffer buf;
-        TRY(buf.append_data(data.data(), data.size()));
-        return buf;
+        return data;
     }
 
     Result<void> File::read(Buffer& buf, usize size)