move definitions and rename some types

[luizirber]

(I'm leaving this as a draft PR because I'm not sure this should be merged, just trying out some ideas)

Main changes

• CompressionFormat and CompressionLevel lost the Compression part, and moved into src/compression.rs. Main consequence on the public API is that you can use

  use niffler::compression;
  niffler::get_writer(..., compression::Format:Gzip, compression::Level::Nine);

  or

  use niffler::compression::{Format, Level};
  niffler::get_writer(..., Format:Gzip, Level::Nine);

• Renamed NifflerError to Error, this way we can do

  use niffler::Error;
  fn get_writer(...) -> Result<..., Error> {}

  instead of having to write NifflerError. And crates using niffler can do niffler::Error instead of niffler::NiflerError.

• Moved read_compression, new_lzma_{encoder,decoder} and new_bz2_{encoder,decoder} tosrc/compression.rs, and made thempub(crate)(so we can call them insrc/lib.rs`).

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (choose_compression_level@a3eda38). Click here to learn what that means. The diff coverage is 68.38%.

@@                     Coverage Diff                     @@
##             choose_compression_level      #11   +/-   ##
===========================================================
  Coverage                            ?   75.64%           
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  Files                               ?        2           
  Lines                               ?      234           
  Branches                            ?       35           
===========================================================
  Hits                                ?      177           
  Misses                              ?       24           
  Partials                            ?       33

Impacted Files	Coverage Δ
src/compression.rs	67.44% <67.44%> (ø)
src/lib.rs	85.71% <73.07%> (ø)

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[natir]

Thanks @luizirber I think all this change are good we can merge it !