More misc cleanup

[rofinn]

• Document Filesystem aliases
• Test consistency with Filesystem behaviour
• Replace Path(fp, segments) with a more general Path(fp; overrides) constructor.

[codecov]

Codecov Report

Merging #86 into master will increase coverage by 0.54%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   87.42%   87.96%   +0.54%     
==========================================
  Files          12       12              
  Lines        1034     1039       +5     
==========================================
+ Hits          904      914      +10     
+ Misses        130      125       -5     

Impacted Files	Coverage Δ
src/system.jl	86.04% <63.63%> (+0.77%)	⬆️
src/path.jl	83.84% <80.00%> (-0.41%)	⬇️
src/aliases.jl	86.66% <100.00%> (+29.52%)	⬆️
src/test.jl	93.31% <100.00%> (ø)
src/posix.jl	85.00% <0.00%> (+7.72%)	⬆️

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