Nested block indexing

[mtfishman]

This enables nested block indexing by generalizing BlockIndex to allow indices that aren't integers, for example:

julia> using BlockArrays

julia> a = mortar([[1, 2], [3, 4]])
2-blocked 4-element BlockVector{Int64}:
 1
 2
 ─
 3
 4

julia> b = mortar([[5, 6], [7, 8]])
2-blocked 4-element BlockVector{Int64}:
 5
 6
 ─
 7
 8

julia> A = mortar([a, b])
2-blocked 8-element BlockVector{Int64, Vector{BlockVector{Int64, Vector{Vector{Int64}}, Tuple{BlockedOneTo{Int64, Vector{Int64}}}}}, Tuple{BlockedOneTo{Int64, Vector{Int64}}}}:
 1
 2
 3
 4
 ─
 5
 6
 7
 8

julia> A[Block(2)[Block(1)]]
2-element Vector{Int64}:
 5
 6

As an alternative to just generalizing BlockIndex, maybe we could define a new type such as GenericBlockIndex and then make BlockIndex a type alias for that with the indices restricted to integers. I don't have a strong opinion about that but this design was simpler and I don't see much advantage one way or another.

Related to #49 and #446, though note that in this PR the nested block structure isn't reflected in the axes, which would be required for more systematic support of nested block structures. This PR is split off from #462.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.05%. Comparing base (a4a0b93) to head (cf14db5).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
+ Coverage   93.81%   94.05%   +0.24%     
==========================================
  Files          19       19              
  Lines        1761     1783      +22     
==========================================
+ Hits         1652     1677      +25     
+ Misses        109      106       -3     

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