Remove einsum

[AntonOresten]

After some thinking in #22, #26, #30, I've decided einsum isn't suited for this package.

In this PR, I also implement some changes I wanted for a v0.2 release, like requiring Val patterns for parse_shape, since it was only type stable without Val if there was no ellipsis.

For a v0.2 release I'm also considering removing EllipsisNotation.jl dependency since it increases the package compile time from 1 second to like 20 seconds because of StaticArrayInterface.jl.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (ddad75d) to head (4d3cf0d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #31      +/-   ##
===========================================
+ Coverage   98.04%   100.00%   +1.95%     
===========================================
  Files          12        11       -1     
  Lines         307       274      -33     
===========================================
- Hits          301       274      -27     
+ Misses          6         0       -6     

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