Replace futures crate with standard library

[tottoto]

Seems to be able to use standard library instead of futures crate at its MSRV 1.75.0.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 84.26%. Comparing base (13bdbf0) to head (1a44d47).

Files	Patch %	Lines
src/lib.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   83.41%   84.26%   +0.84%     
==========================================
  Files           1        1              
  Lines         398      394       -4     
==========================================
  Hits          332      332              
+ Misses         66       62       -4     

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