Implement type consistent ancestor callback

[THargreaves]

An attempt at solving #124.

I think this is probably the cleanest approach without forcing the initial and following states to be of the same type.

I've only implemented the dense ancestor callback for now. Will also do sparse if @charlesknipp likes these changes.

I'm not sure this is a good idea to make all callbacks behave like this in general. For example, the GPU version of this should really store everything a big N x (K+1) array so we would need to force constant types in that case.

For the CPU case though, I think it's reasonable to allow different types.

[github-actions]

SSMProblems.jl/SSMProblems documentation for PR #125 is available at:
https://TuringLang.github.io/SSMProblems.jl/SSMProblems/previews/PR125/

[github-actions]

SSMProblems.jl/GeneralisedFilters documentation for PR #125 is available at:
https://TuringLang.github.io/SSMProblems.jl/GeneralisedFilters/previews/PR125/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.90%. Comparing base (2282c15) to head (b8a27e0).
⚠️ Report is 3 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (2282c15) and HEAD (b8a27e0). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (2282c15)	HEAD (b8a27e0)
GeneralisedFilters	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #125       +/-   ##
===========================================
- Coverage   65.26%   43.90%   -21.37%     
===========================================
  Files          30        5       -25     
  Lines        1212       82     -1130     
===========================================
- Hits          791       36      -755     
+ Misses        421       46      -375     

Flag	Coverage Δ
GeneralisedFilters	?
SSMProblems	43.90% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ 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.