More general checkbounds and blockcheckbounds

[mtfishman]

This adds support for more bounds checking, such as checkbounds(A, Block.(2:3), Block.(2:3)), checkbounds(A, Block(2, 2)[1:3, 1:3]), etc.

It also fixes #458.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.83%. Comparing base (0c73bef) to head (29b5d04).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #469      +/-   ##
==========================================
+ Coverage   93.81%   93.83%   +0.01%     
==========================================
  Files          19       19              
  Lines        1681     1702      +21     
==========================================
+ Hits         1577     1597      +20     
- Misses        104      105       +1     

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