added arithmetic shift operations

[0xflotus]

part 2 of fix #44

[codecov-io]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #47   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           43        45    +2     
  Lines          311       325   +14     
  Branches        49        49           
=========================================
+ Hits           311       325   +14     

Impacted Files	Coverage Δ
source/bits/arithmetic-shift-left.ts	100.00% <100.00%> (ø)
source/bits/arithmetic-shift-right.ts	100.00% <100.00%> (ø)
source/bits/index.ts	100.00% <100.00%> (ø)

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[0xflotus]

oops, I swapped left shift with right shift. Fixed it a few minutes ago.

[FlorianWendelborn]

@0xflotus I merged the circular shift one as it was obvious how it should behave on arbitrary lengths of bits (doesn’t modify length). However, I’m really not sure about this one and if it’s more reasonable to keep the length or to decrease/increase it by amount