Implement Encoding for all Uints

[fjarri]

A part of #1008 that only deals with Encoding:

• Encoding implemented for all Uints. Encoding::Repr for Uint is now an EncodedUint struct instead of an array.
• Macros deriving Encoding are removed.
• Relaxed the bound on the error of Encoding::Repr as TryFrom<&[u8]> from a concrete type to a core::error::Error trait.

[codecov]

Codecov Report

❌ Patch coverage is 91.56627% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.87%. Comparing base (5784b13) to head (ecb405d).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/uint/encoding.rs	90.41%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1016   +/-   ##
=======================================
  Coverage   79.86%   79.87%           
=======================================
  Files         163      163           
  Lines       17709    17737   +28     
=======================================
+ Hits        14143    14167   +24     
- Misses       3566     3570    +4     

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[fjarri]

Note that this can be extended to replace #990 (possibly in a follow-up), and also Encoding can be implemented for BoxedUint.

[fjarri]

What is up with the cross-compile tests? I think there's the same problem in master?

[tarcieri]

Yeah, very strange

[fjarri]

Could it be caused by #1010? I notice they timeouted there too

[tarcieri]

Huh, you're right, guess I didn't notice at the time I merged?

Maybe we should enable some branch protections

[tarcieri]

@fjarri all right, #1017 reverted #1010, can you try rebasing?