unreachable to ord for amount

[cymqqqq]

Background:

Based on #227, there is also an unreachable in the rgb20 amount, so I think maybe we can change to the Ord in order to get rid of unreachable.

Description:

remove the unreachable in the rgb20 amount state change, then use the Ord. Similar to the rgb21 owed fraction state change.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 17.6%. Comparing base (7f637c6) to head (708b207).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/interface/contract.rs	0.0%	8 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #228   +/-   ##
======================================
  Coverage    17.6%   17.6%           
======================================
  Files          37      37           
  Lines        7535    7535           
======================================
  Hits         1323    1323           
  Misses       6212    6212           

Flag	Coverage Δ
rust	17.6% <0.0%> (ø)

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[cryptoquick]

Very nice

[dr-orlovsky]

This must be less, not greater, since we reverse the order neg and add are compared

Suggested change

	Ordering::Greater => AmountChange::Dec(*neg - add),
	Ordering::Less => AmountChange::Dec(*neg - add),

[dr-orlovsky]

I already asked few times to use rebases instead of useless multiple merges of master branch all over the place. Unless there will be a normal commitment history - no master merges, one commit per logical change - I will be just sqash-merging your commits.