add optimized append

[piever]

This fixes the dispatch for push! and append! and adds an optimized append!! method when the iterator being appended is an AbstractVector.

TODO:

• Add tests.

[codecov-io]

Codecov Report

Merging #111 into master will decrease coverage by 1.88%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   91.37%   89.48%   -1.89%     
==========================================
  Files          10       10              
  Lines         371      371              
==========================================
- Hits          339      332       -7     
- Misses         32       39       +7

Impacted Files	Coverage Δ
src/structarray.jl	88.29% <ø> (+1.69%)	⬆️
src/collect.jl	92.95% <71.42%> (-2.64%)	⬇️
src/lazy.jl	84.61% <0%> (-11.54%)	⬇️
src/utils.jl	85.18% <0%> (-7.41%)	⬇️

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