Replace the following index.info fields

  memory_used           # size of statements in memory
  size                  # size of statements on disk
  count                 # number of statements on disk and in memory
  page_count            # number of pages on disk

with

  memory
    rows                # number of statements in memory
    bytes               # size of statements in memory

  disk
    rows                # number of statements on disk
    bytes               # size of statements on disk (unpacked)
    bytes_compressed    # size of statements on disk (packed)
    pages               # number of pages on disk

  rows                  # total number of all statements
  bytes                 # total size of all statements

To achieve that, this patch introduces new classes that can be used for
accounting statements on disk and in memory, vy_stmt_disk_counter and
vy_stmt_counter, and makes vy_slice, vy_run, vy_range, and vy_index use
them instead of counting rows, bytes, and pages directly. The difference
between the two classes is that vy_stmt_counter only accounts rows and
bytes, while vy_stmt_disk_counter also includes bytes_compressed and
pages. The classes will be reused for accounting reads and writes later.

Needed for #1662