Murisi/reverse conversion at source

[murisi]

Describe your changes

Based on the bug fix in #4287. An alternative to #4290 that simplifies the exchanging an amount to some past epoch. If the current epoch is 10 and it is desired to convert an asset from epoch 2 to epoch 6 when only conversions to the latest epoch, 10, are available. Then the following process is now done:

1. Query the conversion from epoch 2 to epoch 10, call it A
2. Query the conversion from epoch 6 to epoch 10, call it B. (If we were converting to epoch 10, then B would equal 0 since that is the conversion from an epoch to itself.)
3. Then return C=A-B. Because of the way the conversion tree is constructed, C is the conversion from epoch 2 to epoch 6 that would have been returned during epoch 6.

So more generally query_allowed_conversion now returns the result of a subtraction A-B. And by definition, the second operand of the subtraction is 0 when converting to the latest epoch, which ultimately means the result is simply A in that case. Additionally, compute_exchanged_amount has been simplified removing the forward and backward conversions.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

[codecov]

Codecov Report

Attention: Patch coverage is 98.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.04%. Comparing base (1465f26) to head (2c580b7).
Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/masp.rs	95.23%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4288   +/-   ##
=======================================
  Coverage   74.03%   74.04%           
=======================================
  Files         345      345           
  Lines      110047   110027   -20     
=======================================
- Hits        81476    81468    -8     
+ Misses      28571    28559   -12     

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[grarco]

lgtm even though personally I prefer the approach of #4290

[murisi]

lgtm even though personally I prefer the approach of #4290

Understood. Thanks for comparing the two PRs!