Some WASM-related refactors

[jplatte]

Not sufficient for compiling the UI crate on WASM, but smaller PRs = better.

Signed-off-by: Jonas Platte [email protected]

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.35%. Comparing base (df4b696) to head (61ac3a6).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/futures.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4491      +/-   ##
==========================================
- Coverage   85.38%   85.35%   -0.03%     
==========================================
  Files         284      284              
  Lines       31885    31885              
==========================================
- Hits        27225    27216       -9     
- Misses       4660     4669       +9     

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[poljar]

Makes sense.