Rename StateHandle to ReadSignal

[lukechu10]

Closes #299

[codecov-commenter]

Codecov Report

Merging #300 (e6822bb) into master (6dd47f4) will not change coverage.
The diff coverage is 90.00%.

@@           Coverage Diff           @@
##           master     #300   +/-   ##
=======================================
  Coverage   75.00%   75.00%           
=======================================
  Files          33       33           
  Lines        4797     4797           
=======================================
  Hits         3598     3598           
  Misses       1199     1199           

Impacted Files	Coverage Δ
packages/sycamore-router/src/router.rs	25.76% <ø> (ø)
packages/sycamore/src/flow.rs	50.00% <ø> (ø)
packages/sycamore/src/lib.rs	100.00% <ø> (ø)
packages/sycamore/src/view.rs	66.03% <ø> (ø)
website/src/main.rs	0.98% <0.00%> (ø)
packages/sycamore-reactive/src/effect.rs	92.09% <100.00%> (ø)
packages/sycamore-reactive/src/iter.rs	97.81% <100.00%> (ø)
packages/sycamore-reactive/src/signal.rs	69.73% <100.00%> (ø)

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