More datatypes

[icweaver]

Adds support for structured (nested) datatypes, and support for ascii and ucs (also outlined in that link), which some newer Roman data files seem to use. This required changing how the datatype field is parsed, so I thought it would be better to have this as a separate PR instead of cramming it into #25.

To add this support, the new AsciiDatatype and Ucs4Datatype types were introduced. These are spiritual variants of Datatype, but since that one is an enum, it seemed like it would be more trouble than it's worth to get them to all subtype a common abstract type.

This PR also includes a grip of tests using the reference files from asdf-standard. I just included a copy of the most recent ones (v1.6.0) in the test directory here for simplicity. On a related note, should the asdf spec version printed here be updated to reflect this?

Could be a good first PR to generalize things to dynamically load different versions directly from that repo instead. Another good one might be to start refactoring src/ASDF.jl now that it is starting to get a little long

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.78%. Comparing base (9810e49) to head (330ac1e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   99.75%   99.78%   +0.03%     
==========================================
  Files           1        1              
  Lines         408      471      +63     
==========================================
+ Hits          407      470      +63     
  Misses          1        1              

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.