Add more methods for mutating arbitrary ranges

[saecki]

This adds the drain and splice methods for EcoVec, the implementation of these have been mostly copy pasted from the standard libaray.

A few more methods for mutating parts of an EcoString that are not at the end are also added:

• remove
• insert
• insert_str

Notably this doesn't implement EcoString::drain, because the returned type would have to be some sort of fusion of the inline vs spilled drain implementation, which seemed a bit inelegant and too much work to implement.

Miri also seems to be happy :)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 80.13468% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.08%. Comparing base (36a2229) to head (aa02e0a).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
src/vec/drain.rs	63.63%	24 Missing ⚠️
src/vec/splice.rs	74.62%	17 Missing ⚠️
src/slice.rs	48.27%	15 Missing ⚠️
src/dynamic.rs	95.58%	3 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
- Coverage   88.40%   84.08%   -4.32%     
==========================================
  Files           3        6       +3     
  Lines        1095     1257     +162     
==========================================
+ Hits          968     1057      +89     
- Misses        127      200      +73     

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[saecki]

I think some more test cases for drain and splice wouldn't hurt. I'll add them later today.