Fix contexts inside effects

[lukechu10]

Fixes #308

[codecov-commenter]

Codecov Report

Merging #310 (c6a31b9) into master (eda8536) will increase coverage by 0.86%.
The diff coverage is 93.57%.

@@            Coverage Diff             @@
##           master     #310      +/-   ##
==========================================
+ Coverage   72.34%   73.21%   +0.86%     
==========================================
  Files          38       38              
  Lines        5222     5313      +91     
==========================================
+ Hits         3778     3890     +112     
+ Misses       1444     1423      -21     

Impacted Files	Coverage Δ
packages/sycamore/src/view.rs	66.03% <ø> (ø)
packages/sycamore-reactive/src/effect.rs	91.04% <86.11%> (+3.09%)	⬆️
packages/sycamore-reactive/src/lib.rs	95.77% <88.88%> (+1.74%)	⬆️
packages/sycamore/src/context.rs	99.10% <98.30%> (-0.90%)	⬇️
packages/sycamore-reactive/src/context.rs	82.00% <100.00%> (+2.00%)	⬆️
packages/sycamore-reactive/src/iter.rs	97.81% <100.00%> (ø)

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