Reduce allocations and dynamic dispatch in effects and memos

[lyphyser]

This series of commits removes unnecessary allocations in effects and memos and makes calling effect callbacks only do a single dynamic dispatch rather than two.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.07 🎉

Comparison is base (3885a9e) 60.52% compared to head (41ff2b6) 60.60%.

❗ Current head 41ff2b6 differs from pull request most recent head 9acfa5d. Consider uploading reports for the commit 9acfa5d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #599      +/-   ##
==========================================
+ Coverage   60.52%   60.60%   +0.07%     
==========================================
  Files          56       56              
  Lines        9380     9399      +19     
==========================================
+ Hits         5677     5696      +19     
  Misses       3703     3703              

Impacted Files	Coverage Δ
packages/sycamore-reactive/src/effect.rs	99.44% <100.00%> (+0.03%)	⬆️
packages/sycamore-reactive/src/memo.rs	100.00% <100.00%> (ø)
packages/sycamore-reactive/src/signal.rs	86.38% <100.00%> (ø)

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[lukechu10]

Supserseded by #626