fix type stability in autocov and add a unit test

[vandenman]

Fixes #269

I tried to make the test a bit less copy-pasta with

for property in (:cross, :m1, :m2, :lag, :wlag, :v)
    @eval begin
        @test (@inferred ((o) -> o.property)(o)) === o.property
    end
end

but unfortunately, that doesn't work since @eval always works in global scope, so I just copy-pasted it.

You can test that this solves the issue with _fit! by running:

@code_warntype OnlineStats._fit!(AutoCov(10), randn(100))

Suggestions, comments, and feedback are welcome!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7813dc2) 68.47% compared to head (842d085) 68.01%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
- Coverage   68.47%   68.01%   -0.46%     
==========================================
  Files          21       21              
  Lines        1973     1973              
==========================================
- Hits         1351     1342       -9     
- Misses        622      631       +9     

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