Fix some bugs

[jakobnissen]

• Make sure == doesn't dispatch to memcmp if the mem's eltype is a union, since in that case the array's bits can be the same but its content differ due to the type metadata array. Also, test for this
• However, ensure that =='ing immutable and mutable arrays does dispatch to memchr, even if the two arrays type differ in mutability
• Fix an edge case in split_unaligned when no part of the array is aligned
• Minor refactor, using the internal truncate functions more. This change allows some branches to be omitted.

[codecov]

Codecov Report

Attention: Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.55%. Comparing base (83b4218) to head (d7bc579).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/basic.jl	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
- Coverage   98.34%   97.55%   -0.80%     
==========================================
  Files           5        5              
  Lines         242      245       +3     
==========================================
+ Hits          238      239       +1     
- Misses          4        6       +2     

Flag	Coverage Δ
unittests	97.55% <88.23%> (-0.80%)	⬇️

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