Add dedicated BlockIterators

[lkdvos]

This PR adds some dedicated iterators for going through the blocks of a single tensor.
This replaces the previous default implementation in terms of Iterators.map, in order to have more type stability (well-defined eltype and length), and the option to index into the iterators.
This should otherwise not change any external functionality (and in particular, it re-enables the syntax blocks(t)[c], which worked before v0.13) so I think it could be considered non-breaking.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 9 lines in your changes missing coverage. Please review.

Project coverage is 82.17%. Comparing base (660bdf7) to head (fa794bf).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/tensors/diagonal.jl	66.66%	4 Missing ⚠️
src/tensors/abstracttensor.jl	33.33%	2 Missing ⚠️
src/tensors/adjoint.jl	80.00%	2 Missing ⚠️
src/tensors/blockiterator.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
- Coverage   82.18%   82.17%   -0.02%     
==========================================
  Files          42       43       +1     
  Lines        5384     5424      +40     
==========================================
+ Hits         4425     4457      +32     
- Misses        959      967       +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.