Make create_ref only allow T: 'static

[lukechu10]

Fixes #494

Closes #400 (basically makes the code pattern impossible)

This essentially makes create_ref only work on 'static types which completely prevents the issue mentioned in #494.

Unfortunately this makes create_ref much less powerful, but in practice, it seems like this won't be too big of a setback. In fact, none of the examples needed to be modified.

[codecov]

Codecov Report

Base: 64.69% // Head: 64.60% // Decreases project coverage by -0.08% ⚠️

Coverage data is based on head (b1df775) compared to base (de737e5).
Patch coverage: 63.49% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #519      +/-   ##
==========================================
- Coverage   64.69%   64.60%   -0.09%     
==========================================
  Files          52       52              
  Lines        8537     8533       -4     
==========================================
- Hits         5523     5513      -10     
- Misses       3014     3020       +6     

Impacted Files	Coverage Δ
packages/sycamore-router/src/router.rs	27.88% <0.00%> (ø)
packages/sycamore/src/motion.rs	0.00% <0.00%> (ø)
packages/sycamore-reactive/src/arena.rs	100.00% <100.00%> (ø)
packages/sycamore-reactive/src/effect.rs	99.31% <100.00%> (ø)
packages/sycamore-reactive/src/iter.rs	97.78% <100.00%> (ø)
packages/sycamore-reactive/src/lib.rs	93.65% <100.00%> (-0.60%)	⬇️
packages/sycamore-reactive/src/memo.rs	100.00% <100.00%> (ø)
packages/sycamore-reactive/src/signal.rs	87.30% <100.00%> (+0.34%)	⬆️
packages/sycamore/src/builder.rs	21.66% <100.00%> (ø)
packages/sycamore/src/suspense.rs	94.61% <100.00%> (+0.04%)	⬆️
... and 2 more

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