Changes:
* Make `NewReader` method useful. It was impossible to use it, because closer was always nil.
* ReadSymbols, TOC and ReadOffsetTable are not public functions (used by Thanos).
* decbufXXX are now functions.
* More verbose errors.
* Removed unused crc32 field.
* Some var name changes to make it more verbose:
* symbols -> allocatedSymbols
* symbolsSlice -> symbolsV1
* symbols -> symbolsV2
*
* Pre-calculate symbolsTableSize.
* Initialized symbols for Symbols() method with valid length.
* Added test for Symbol method.
* Made Decoder LookupSymbol method public. Kept Decode public as it is useful as helper from index package.
Signed-off-by: Bartek Plotka <bwplotka@gmail.com>
