opt: use the std math/bits routine to count the bit length

[0-haha]

1. Are you opening this pull request for bug-fixes, optimizations or new feature?

optimizations

2. Please describe how these code changes achieve your intention.

bits.Len() is optimized by the compiler as an intrinsic function, which uses an inline BSRQ instruction. This is significantly faster than using the for loop.

See the corresponding assembly code https://godbolt.org/z/rTebv3q7c

3. Please link to the relevant issues (if any).

no

4. Which documentation changes (if any) need to be made/updated because of this PR?

no

4. Checklist

• I have squashed all insignificant commits.
• I have commented my code for explaining package types, values, functions, and non-obvious lines.
• I have written unit tests and verified that all tests passes (if needed).
• I have documented feature info on the README (only when this PR is adding a new feature).
• (optional) I am willing to help maintain this change if there are issues with it later.

[codecov]

Codecov Report

Base: 76.46% // Head: 76.46% // No change to project coverage 👍

Coverage data is based on head (87d6195) compared to base (0f41b37).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #437   +/-   ##
=======================================
  Coverage   76.46%   76.46%           
=======================================
  Files          15       15           
  Lines        1500     1500           
=======================================
  Hits         1147     1147           
  Misses        271      271           
  Partials       82       82           

Flag	Coverage Δ
unittests	76.46% <ø> (ø)

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[panjf2000]

Thanks~