Use length instead of Wrappers.Length

[fingolfin]

Works because we have a method

Base.length(x::GapObj) = Wrappers.Length(x)::Int

and because a few GAP wrappers are strengthened to assert their
return types, thus making the code which calls length more
type stable.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.42%. Comparing base (bbdbe2b) to head (e233942).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/wrappers.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1117      +/-   ##
==========================================
- Coverage   75.27%   74.42%   -0.86%     
==========================================
  Files          55       55              
  Lines        4668     4668              
==========================================
- Hits         3514     3474      -40     
- Misses       1154     1194      +40     

Files with missing lines	Coverage Δ
src/constructors.jl	97.56% <100.00%> (ø)
src/gap_to_julia.jl	90.44% <100.00%> (ø)
src/wrappers.jl	97.18% <66.66%> (ø)

... and 2 files with indirect coverage changes