Support odd Codable behaviors

[gwynne]

These changes are now available in 4.5.2

Currently, LeafEncoder throws a fatal error if one attempts to encode more than one object into the same keyed or unkeyed encoding container. Doing so is incorrect use of the Codable API, but it is nonetheless a semi-common practice, and is also the only practical way to accomplish some things. This change lifts the restriction and allows such encoding to work as expected.

Also bumps the package's Swift version requirement to 6.1 and converts the tests to SwiftTesting.

[codecov]

Codecov Report

❌ Patch coverage is 35.71429% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.34%. Comparing base (42e8274) to head (4681ef7).

Files with missing lines	Patch %	Lines
Sources/Leaf/LeafEncoder.swift	35.71%	45 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (42e8274) and HEAD (4681ef7). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (42e8274)	HEAD (4681ef7)
	6	5

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #242      +/-   ##
==========================================
- Coverage   76.95%   67.34%   -9.62%     
==========================================
  Files           5        5              
  Lines         243      297      +54     
==========================================
+ Hits          187      200      +13     
- Misses         56       97      +41     

Files with missing lines	Coverage Δ
Sources/Leaf/LeafEncoder.swift	52.43% <35.71%> (-11.69%)	⬇️

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[MahdiBM]

Looks good as far as I can tell.