Add .map utility for ReadSignal as a wrapper around create_memo

[lukechu10]

No description provided.

[codecov-commenter]

Codecov Report

Merging #326 (68fbd07) into master (b9c3089) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
- Coverage   73.20%   73.15%   -0.06%     
==========================================
  Files          40       40              
  Lines        5431     5435       +4     
==========================================
  Hits         3976     3976              
- Misses       1455     1459       +4     

Impacted Files	Coverage Δ
packages/sycamore-reactive/src/signal.rs	67.94% <0.00%> (-1.79%)	⬇️

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