Pcodec

Reduced minimum version requirement for `half` crate.

* New format version (2)
* API breaking changes
* Replaced `FloatMultSpec` and `IntMultSpec` with `mode_spec: ModeSpec` in `ChunkConfig`.
* Removed `auto_delta_encoding_order` function.
* API additions
* Added support for 16-bit data types.
* Added `LatentDescriber` API, accessible by `NumberLike::get_latent_describer`.
* Improvements
* Added `FloatQuant` mode and automatic detection for it, improving compression ratio on quantized float data.
* Increased compression speed for high compression levels slightly.

Python_v0.2.0
* Added support for 16-bit types.
* Replaced separate `int_mult_spec` and `float_mult_spec` configs with`mode_spec: ModeSpec`.
* Upgraded Rust implementation from 0.2.5 to 0.3.0.

Python_v0.1.1
Upgraded Rust implementation from 0.1.3 to 0.2.5.

Fixed a bug in v0.2.4 that caused a panic during histogram computation under certain conditions.

* Made int mult detection more statistically principled, reducing false positive rate.
* Improved compression speed by sorting only partially to compute the histogram. This is 10-50% faster at default compression level, but 300+% faster at low compression levels.
* Yanked because a bug (causing a panic) was discovered right after publishing.

No real changes, but bumped `better_io` version to 0.1.0.

* Fixed https://github.com/mwlon/pcodec/issues/178