Fix to_index for CartesianIndices and first for LazyAxis

[wangl-cc]

Fix #308, #309

[codecov]

Codecov Report

Merging #310 (6af3483) into master (de94815) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #310   +/-   ##
=======================================
  Coverage   91.13%   91.13%           
=======================================
  Files           9        9           
  Lines        1398     1398           
=======================================
  Hits         1274     1274           
  Misses        124      124           

Impacted Files	Coverage Δ
src/axes.jl	92.05% <0.00%> (ø)
src/indexing.jl	86.89% <100.00%> (ø)

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[Tokazama]

Thanks! I'll try to get some more exhaustive tests for LazyAxis in soon so we can ensure this sort of thing doesn't happen anymore.