Codec trait and array refactor

[LDeakin]

This is a major refactor of codec traits and the array structure.
Previously, various codec and array methods had three variants:

1. <method>_opt(..., parallel: bool) supporting optional parallelism
2. <method>(...), serial
3. par_<method>(...) , parallel

Now, there are just two variants:

1. <method>_opt(..., options: &SomeOptionsStruct)
2. <method>(...), default options (maximum concurrency)

This new API provides finer-grained control of the number of concurrent operations executed within an encode/decode method.
Most internal operations use the _opt API. The efficient/maximum concurrency of codecs can be queried (presently, this may not be accurate for many codecs).

This has the potential to improve performance and reduce memory usage. For example, a call of Array::retrieve_chunks which decodes multiple shards could recognise that the inner chunks can be decoded with massive parallelisation, and thus only decode one shard at a time. This behaviour is not yet implemented, but will follow shortly.

[codecov]

Codecov Report

Attention: 786 lines in your changes are missing coverage. Please review.

Comparison is base (5eab757) 77.57% compared to head (2433c2a) 75.71%.

Files	Patch %	Lines
src/array/array_async_readable.rs	0.00%	188 Missing ⚠️
src/array/array_async_readable_writable.rs	0.00%	135 Missing ⚠️
src/array/array_sync_readable.rs	32.00%	85 Missing ⚠️
src/array/array_async_writable.rs	0.00%	72 Missing ⚠️
src/array/array_sync_readable_writable.rs	29.41%	72 Missing ⚠️
src/array/array_sync_writable.rs	5.45%	52 Missing ⚠️
src/array/codec.rs	68.59%	38 Missing ⚠️
src/array/codec/array_to_bytes/codec_chain.rs	41.37%	34 Missing ⚠️
...rc/array/codec/array_to_bytes/bytes/bytes_codec.rs	17.39%	19 Missing ⚠️
...ay/codec/array_to_bytes/sharding/sharding_codec.rs	91.87%	13 Missing ⚠️
... and 12 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   77.57%   75.71%   -1.87%     
==========================================
  Files         123      124       +1     
  Lines       17606    17878     +272     
==========================================
- Hits        13658    13536     -122     
- Misses       3948     4342     +394     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.