feat(Algebra\ContinuedFractions): add partNums! and partDens!

[emlis42]

This PR adds partNums! and partDens!, which provide infinite stream representations for the sequences of partial numerators and denominators of a generalized continued fraction.

[github-actions]

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[github-actions]

PR summary 6d6d92b454

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ partDens!
+ partNums!

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

---

No changes to strong technical debt.

No changes to weak technical debt.