Fix type stability of callback functions

[odow]

Closes #518, but I'm not overly enthused about it.

I need to do some proper testing.

[codecov]

Codecov Report

❌ Patch coverage is 83.87097% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.15%. Comparing base (077843b) to head (394ed3c).

Files with missing lines	Patch %	Lines
ext/IpoptMathOptInterfaceExt/MOI_wrapper.jl	83.87%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #519      +/-   ##
===========================================
- Coverage   100.00%   99.15%   -0.85%     
===========================================
  Files            5        5              
  Lines         1164     1178      +14     
===========================================
+ Hits          1164     1168       +4     
- Misses           0       10      +10     

☔ 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.