Custom Error Types, Serde Support, Improved #![no_std] Support

[poshcoe]

Fixes #8 & #9

• Custom error types have been implemented, removing the anyhow dependency.
• Serialisation of public types is now optionally derived via serde.
• Methods using String are now usable on #![no_std] via the alloc feature.
• New write_iso methods are available for writing static buffers with ISO formatted datetimes.

Note:
Custom error types implement core::error::Error for #![no_std], requiring the error_in_core feature, which is stabilising in Rust 1.81. Until 1.81 is released on September 5th, the core::error::Error implementations are behind the nightly feature.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (68796d7) to head (b9756d8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main       #10    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            3         4     +1     
  Lines          686       862   +176     
==========================================
+ Hits           686       862   +176     

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