Modified the backend code of the function scan() to improve efficiency.

[learningMalanya]

Modified the source code for the function scan() for improving efficiency:

• Modified scan() and scan_perms_lite(), such that single-trait scan ("Null" assumption) will now be performed through vectorized operations in scan_perms_lite(), not by scan_null() anymore.
• No change to the functions' API.

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 76.86%. Comparing base (b78d3ad) to head (bd5eef3).

Files	Patch %	Lines
src/scan.jl	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #100      +/-   ##
==========================================
- Coverage   77.08%   76.86%   -0.22%     
==========================================
  Files          12       12              
  Lines         733      722      -11     
==========================================
- Hits          565      555      -10     
+ Misses        168      167       -1     

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