Faster extrema calculations for large series

[BioTurboNick]

Due to type instability, the expand_extrema methods were very slow. Adding a type assert fixes that and greatly improves the performance of plotting large series.

Test example:

const x6 = rand(1000000, 10) # several large series
plot(x6);

Before: 1.004 s (19999416 allocations: 457.95 MiB)

After: 209.896 ms (4526 allocations: 152.85 MiB)

(Numbers both include the effect of an unmerged RecipesPipeline PR, but the impact is the same)

[codecov]

Codecov Report

Base: 80.87% // Head: 80.87% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (50a6df4) compared to base (539e5da).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4412   +/-   ##
=======================================
  Coverage   80.87%   80.87%           
=======================================
  Files          30       30           
  Lines        7439     7440    +1     
=======================================
+ Hits         6016     6017    +1     
  Misses       1423     1423           

Impacted Files	Coverage Δ
src/axes.jl	86.13% <100.00%> (ø)
src/pipeline.jl	96.12% <100.00%> (+0.01%)	⬆️

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[t-bltg]

lgtm