Support indexing abstractions for containers

[timholy]

For objects that support obj[i], also support eachindex
and (in some cases) axes as mechanisms to query
the valid indexes.

[codecov]

Codecov Report

Base: 68.12% // Head: 64.16% // Decreases project coverage by -3.95% ⚠️

Coverage data is based on head (7cbbc74) compared to base (97561bb).
Patch coverage: 60.00% of modified lines in pull request are covered.

❗ Current head 7cbbc74 differs from pull request most recent head 55c173b. Consider uploading reports for the commit 55c173b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #677      +/-   ##
==========================================
- Coverage   68.12%   64.16%   -3.96%     
==========================================
  Files          32       32              
  Lines        2729     2648      -81     
==========================================
- Hits         1859     1699     -160     
- Misses        870      949      +79     

Impacted Files	Coverage Δ
src/lists.jl	74.78% <42.85%> (-0.67%)	⬇️
src/container.jl	76.78% <100.00%> (-1.40%)	⬇️
src/layout.jl	80.85% <100.00%> (+0.20%)	⬆️
src/toolbar.jl	90.47% <100.00%> (+0.47%)	⬆️
src/precompile.jl	0.00% <0.00%> (-100.00%)	⬇️
src/gtktypes.jl	33.33% <0.00%> (-45.62%)	⬇️
src/GLib/GLib.jl	66.66% <0.00%> (-15.16%)	⬇️
src/GLib/gtype.jl	63.13% <0.00%> (-15.01%)	⬇️
src/events.jl	45.91% <0.00%> (-9.19%)	⬇️
src/GLib/gvalues.jl	83.87% <0.00%> (-6.27%)	⬇️
... and 6 more

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[coveralls]

Pull Request Test Coverage Report for Build 3587094088

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 6 of 9 (66.67%) changed or added relevant lines in 4 files are covered.
• 60 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.3%) to 64.724%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lists.jl	3	6	50.0%

Files with Coverage Reduction	New Missed Lines	%
src/GLib/signals.jl	6	73.0%
src/lists.jl	54	74.38%

Totals
Change from base Build 3217240595:	1.3%
Covered Lines:	1699
Relevant Lines:	2625

---

💛 - Coveralls