Move isassigned boundscheck to FSA wrapper

[jakobnissen]

In Base.boundscheck, instead of forwarding boundscheck to the inner vector, do the boundscheck on the wrapper FSA, and then call inner isassigned with @inbounds.
This allows correct results for unsafe inner arrays which does not, or cannot boundscheck.
In general, it would be preferred for FSA to check indices where possible, since the FSA wrapper "owns" the indices.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.85%. Comparing base (147851b) to head (b13064e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #188      +/-   ##
==========================================
+ Coverage   92.78%   92.85%   +0.06%     
==========================================
  Files           5        5              
  Lines         208      210       +2     
==========================================
+ Hits          193      195       +2     
  Misses         15       15              

☔ 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.

[nsajko]

Looking at @code_native, this change does not appear to cause significant changes for the generated code. At least on my platform, Intel AMD64.

Out of courtesy, we should make sure not to commit with a commit message pinging Github user inbounds.

[jakobnissen]

Good sentiment, but @inbounds is a dummy GitHub account set up by JuliaLang - see https://github.com/orgs/JuliaLung/people 😄