fix!: report unreadable tables as read failures

[nepinhum]

Closes #16

Changes

• TableReader.get() returns !TableHit (found / value / kt) and
  propagates decode_block_handle, read_block (checksum and decompression),
  decode_block and parse_internal_key. The index search is split into
  block_index(), which cannot fail and block_handle(), which reports an index entry
  that will not decode.
• DB.get() returns !Lookup (found / value), DB.has() returns !bool
  and both db.table(f.num) or { continue } sites propagate.
• A Bloom filter rejection and a block search that finds no matching key stay
  plain absence.

The finding suggests !?[]u8. V has no !?T, so presence rides inside the result as Lookup.found.
Same three outcomes: found, not found, read failure.

Breaking

// before
if value := db.get(key, leveldb.ReadOptions{}) {
      println(value.bytestr())
}

// after
r := db.get(key, leveldb.ReadOptions{})!
if r.found {
      println(r.value.bytestr())
}

CHANGELOG.md carries the same note.

Tests

• v test . [passed]
• v run examples/smoke/main.v

[nepinhum]

@xRookieFight can you review?

[nepinhum]

windows ci fails with:

& macos tests passed the checks

[xRookieFight]

windows ci fails with:

& macos tests passed the checks

@medvednikov