Nested slicing operations for merging blocks

[mtfishman]

Fixes #359.

This introduces syntax for merging blocks by indexing with nested vectors of Block{1}, BlockRange{1}, BlockIndexRange{1}, etc. The implementation is very similar to #445.

As an example:

julia> using BlockArrays

julia> A = BlockArray(randn(8, 8), [2, 2, 2, 2], [2, 2, 2, 2])
4×4-blocked 8×8 BlockMatrix{Float64}:
  0.163738   -0.859246  │  -0.399256  -0.143974   │  -0.812783   0.469877  │   0.0631785   1.56266  
 -0.209473    1.23701   │   0.479233   0.219572   │  -0.548844   1.363     │   0.795119   -0.169581 
 ───────────────────────┼─────────────────────────┼────────────────────────┼────────────────────────
  1.34493    -0.846004  │   1.32566    0.470043   │   0.650647   0.617614  │  -0.0926643   0.550304 
  1.20779    -0.528143  │   1.09621   -0.0868135  │  -0.178719  -0.105456  │   0.243687   -1.31676  
 ───────────────────────┼─────────────────────────┼────────────────────────┼────────────────────────
  1.201       1.48691   │   0.264342   0.696739   │  -0.86067   -0.608839  │  -0.082433   -0.0931981
  0.674171   -1.23494   │   0.124443   1.94983    │   0.929236  -1.62257   │  -0.693029   -0.0212493
 ───────────────────────┼─────────────────────────┼────────────────────────┼────────────────────────
  0.0174839   1.65394   │  -0.851706  -1.23722    │   1.63834    0.158663  │   1.27422    -2.37923  
  1.28377     0.364039  │   0.564611  -1.346      │  -0.619627   0.240933  │   0.270664   -0.593572 

julia> A[[[Block(1), Block(2)], [Block(3), Block(4)]], [[Block(1), Block(2)], [Block(3), Block(4)]]]
2×2-blocked 8×8 BlockedMatrix{Float64}:
  0.163738   -0.859246  -0.399256  -0.143974   │  -0.812783   0.469877   0.0631785   1.56266  
 -0.209473    1.23701    0.479233   0.219572   │  -0.548844   1.363      0.795119   -0.169581 
  1.34493    -0.846004   1.32566    0.470043   │   0.650647   0.617614  -0.0926643   0.550304 
  1.20779    -0.528143   1.09621   -0.0868135  │  -0.178719  -0.105456   0.243687   -1.31676  
 ──────────────────────────────────────────────┼──────────────────────────────────────────────
  1.201       1.48691    0.264342   0.696739   │  -0.86067   -0.608839  -0.082433   -0.0931981
  0.674171   -1.23494    0.124443   1.94983    │   0.929236  -1.62257   -0.693029   -0.0212493
  0.0174839   1.65394   -0.851706  -1.23722    │   1.63834    0.158663   1.27422    -2.37923  
  1.28377     0.364039   0.564611  -1.346      │  -0.619627   0.240933   0.270664   -0.593572 

julia> A[[[Block(1)[2:2], Block(2)[2:2]], [Block(3)[2:2], Block(4)[2:2]]], [[Block(1)[2:2], Block(2)[2:2]], [Block(3)[2:2], Block(4)[2:2]]]]
2×2-blocked 4×4 BlockedMatrix{Float64}:
  1.23701    0.219572   │   1.363     -0.169581 
 -0.528143  -0.0868135  │  -0.105456  -1.31676  
 ───────────────────────┼───────────────────────
 -1.23494    1.94983    │  -1.62257   -0.0212493
  0.364039  -1.346      │   0.240933  -0.593572 

It feels unfortunate that the implementation has to be done in this way by catching the various cases manually with dispatch, but I can't think a better way (besides introducing a custom slicing operation).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.95%. Comparing base (cc32221) to head (3b0ce73).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
+ Coverage   93.90%   93.95%   +0.04%     
==========================================
  Files          19       19              
  Lines        1707     1721      +14     
==========================================
+ Hits         1603     1617      +14     
  Misses        104      104              

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