improve `Vararg` checking

[oscardssmith]

This code really shouldn't be messing around with function signatures, but if it is, it should do so better. This allows recognizing things like g(x::Vararg{Vector{T}, N}) where {N, T}.

[codecov[bot]]

Codecov Report

Merging #457 (d9d1585) into master (68f2017) will decrease coverage by 2.55%. The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #457      +/-   ##
==========================================
- Coverage   49.07%   46.52%   -2.55%     
==========================================
  Files          46       46              
  Lines        3519     3527       +8     
==========================================
- Hits         1727     1641      -86     
- Misses       1792     1886      +94     

Impacted Files	Coverage Δ
src/utils.jl	66.45% <90.00%> (+4.36%)	:arrow_up:

... and 27 files with indirect coverage changes

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[ChrisRackauckas]

This needs a test.

[ChrisRackauckas]

Test failure is real.

[ChrisRackauckas]

bump